A planet with an early civilization (Middle age like) is doomed to be destroyed by an unstoppable event. So an advanced civilization decides to do a planetary evacuation to save the species and their culture, which is staged as an event based on the mythological beliefs of the early civilization to cover their tracks. However, is there a reason to transfer the early civilization into an artificial habitat like an O'neill Cylinder rather than move them to a different planet?
Convenience, due to multiple and maybe concurring factors, for example:
- the cylinder can be brought closer to the rescue place, easing logistic on the evacuation. One thing is moving a lot of people to few light seconds away, another thing if that distance increases.
- the planet might have a better use for the rescuer than hosting refugees. What's the point of saving them if then they get threatened again by mines/real estate agencies/etc.
- the cylinder can be tailored to contain some features relevant to the mythology of the rescued people
You need an artificial habitat no matter what.
The nearest suitable planet for this species is many light years away. Some delicate aspect of this species' biology makes cryogenic preservation or other forms of "stasis" unsuitable, meaning that all members of the species will be awake for the entire trip, which could last decades, centuries, or more. Whether or not you ultimately plan to re-home this civilization on a planet, you're going to need an artificial habitat where potentially generations of individuals will live and die. No matter what the long-term plan is, the first step will involve transferring many individuals to a large-scale artificial habitat.
It's a bit like asking why you'd choose an RV to do a long-distance family road trip - it's worthwhile to have on a long enough trip, regardless of your final destination.
Biologically incompatible species
There is an incompatibility between available planets and the biology of some/all of the species required for the primitive civ to survive. A constructed habitat is a simple (though likely not easy) solution to that problem.
Ease of access
They might want to do more stuff with the rescued civilization during and after rescue, which depending on the morality of the culture of the rescuers could range from trained enslavement, through extreme-mild research, and over to something like benign integration into the overall larger society.
This is a bit out of bound for the question (you specifically asked about practical reasons, but a habitat could be a practical solution to a political problem (for example)). The faction that wants to save the planet ends up using a habitat for the simple reason that they cannot get access to anything else within the time frame required. Or perhaps they were sponsored by ACME Habitats inc. etc.
Wishes from the primitive civ
Again slightly out of scope, but they might have contacted the primitive civ in some manner and put out the options for them, and the choice was to go with the habitat.
Let's take another point of view on this situation : a primitive civilization will be relocated in order to keep them from extinction. Sure, you could bring them to some planet that already has everything they'll need to start fresh and leave them be. But what about making this "heroic deed" profitable as well?
By putting this civilization in a controled, fully customizable environment, you can make them a primary source of endless, cheap entertainment. People love medieval stories, reality shows and so on. With this solution, there's no need to use actors, fictions and the likes.
You make the environment from scratch, so anything like cameras, microphones and any other way to spy on them is easy to implement. You can even prebuild some habitats for them, in order to have full access to their full life, 24/7.
What about going further? If you have control over the entire environment, you could also create just about any kind of situation you couldn't easily create in a natural world. Having control over the water and food supply, the quality of the soil... You could create wars with minimal effort. Is a group of individuals foiling your plans and potentially ruining your show? Remove them using those convenient "maintenance" systems. With this kind of control, any individual is a "character" you can use for entertainment and marketing purposes.
Sure, you could do all that on a natural planet, but it would be much easier to handle a world made for that purpose, rather than having to repurpose an already existing ecosystem. It's all about profit, and what the public wants to watch.
Why rescue them?
The aliens value the diversity of civilizations in the galaxy. They want to preserve as many as possible, not as dry databases but as actual, thriving, developing cultures.
The aliens also value biodiversity. A civilization might rate slightly higher than non-sentient animals, but they'd rather preserve the animals, too. Any planet that would be a shirt-sleeve environment for the primitives is going to have native animals.
They cannot terraform in an instant.
The aliens have FTL travel, and the ability to build a big habitat on short notice, but terraforming an entire planet is something else.
So they want to resuce the civilization, but they don't want to disturb other biospheres if they can help it. Fortunately they can help it -- they replace the doomed planet with an artificial dome.
It's on the way
Due to speed limits, the nearest suitable planet is thousands of years away. So whatever vessel you use to transport the civilization, it needs to be a self-sustaining ecosystem in its own right.
Enter Professor Gerard K. O'Neill and his flabbergasting cylindrizer.
Perhaps the cylinder is a big space ship that slowly sails to the new home planet.
Or perhaps the cylinder stays where it is. Once they built the cylinder the aliens didn't see the point of relocating the civilization a second time. The cylinder is as good a home world as any.
Practicality & Simplicity.
The following assumptions are made in this answer:
The advanced race has time to construct at least one cylinder before the extinction event occurs and the science/engineering involved in constructing an O'Neill Cylinder is well understood.
The aliens have many ships but they are also spread across many systems and those ships have other responsibilities beyond just trying to rescue this one species, including looking after the needs of their own people.
They can't save everyone, even on a planet with a population density equating to that of Earth in the Middle Ages. But they believe they have a moral imperative to save as many of the species and as much of their biosphere presumably as they can. Perhaps some kind of 'life is rare and intelligent life even rarer' perspective which makes the medieval aliens precious in the eyes of the advanced aliens.
Assuming these starting conditions apply and the alien race that needs saving lives on an Earth like planet in metal rich, Sol type star this means the solar system is likely to have abundant resources in space for the construction a cylinder. So they have ample raw materials right where they need them.
This means nothing needs to be transported into the solar system for construction of the cylinder except perhaps for some initial construction / printing and mining equipment. (And maybe not even much of that since the aliens can 'bootstrap' rapidly from a tiny industrial base very quickly.) Say one ships worth of equipment.
So once it arrives (assuming they don't already have it) They just locate the largest metallic asteroid they can find and press the 'start' button.
Once construction commences no other effort is required by the aliens beyond perhaps construction of the 'drive' they want to attach to the cylinder. (Since there's no reference to how the aliens travel between the stars I've just ignored this issue.)
As soon as the cylinder is airtight the aliens can start building the biosphere while work on other essential systems (like the drive) is ongoing. So they can start moving plants and animals up from the surface (with locals reporting lots of mysterious lights in the sky).
Lastly they kidnap a selection of medieval aliens and explain what has happened to them if not why.
A planet? Good heavens, why would you inflict so crude and plebian a dwelling place on them?
Of course they were living on one before, but given that you are removing them anyway, what conceivable reason is there to plant them on one of those vast, uncontrollable, and quite dangerous places? The best science in the world can not make the planet quite safe. The primitives might face an ice age, or a drought! No, much more humane to bring them into a place where the habitat is harmoniously controlled by wise minds.
Humans won't see reason about environmental destruction. They won't stop competing to dig up resources to kill each other. The aliens can't kill them - it would be unethical - and they can't just step in and clean up the planet (the humans would abuse their resources even more, reverse engineer and otherwise exploit the alien technology, and destroy nearby habitable worlds). So there's no way to keep the Earth from dying.
Problem is, the honey badgers will die with it. And the aliens know the honey badgers have responded to recent ecological turmoil by evolving sentience and starting the rudiments of civilization. (They may be more trouble than humans, but they're not sure to work together to destroy their new planet). The honey badgers have to be rescued.
Now the aliens could dump honey badgers on a freshly terraformed planet, easy peasy. Except... the species will be denied their normal development. They should grow up in a "natural" environment, and that means not just rich wild ecosystems and a perfect color of sky blue, but running the 3D printers and completely refurbishing the geology of the target planet to show the Descent of Honey Badger with reasonable authenticity. (Some fringe creationist groups will insist there are "missing links" between the time points of the simulated strata, but they would have done so even on a natural homeworld, most likely)
Result: honey badgers spend a subjective 40 days and 40 nights zooming from one homeworld to another at a very fast sublight speed on an advanced megastructure, arriving thousands of years in the future to be transferred to a perfectly authentic fake homeworld the aliens have made in the meanwhile.
Capitalism, Colonisation and "Ethics"
You want to displace this population and take their planet for your own. Ethical rules and other such inconveniences prevent you from wiping them out militarily. They've equally proven inconveniently resistant to all the diseases you've tried to give them.
So you generate a cataclysm
Offer them an escape, an artifical habitat away from the unfortunate apocalypse that's preventing them from remaining on the planet while you rebuild their environment. You've ticked all the boxes, you haven't wiped them out, you haven't used the army, they have gone willingly, you haven't had to give up another valuable planet in exchange.
Unfortunately by the time their planet is suitably hospitable for a primitive tribe, their lands have all been taken over by your industrial expansion. After all, it was a hostile environment with no existing population, so why not. However they're welcome to remain on the habitat with safe controlled weather and no dangerous extremes, no drought, no floods, and no cost to themselves.
A round of cost cutting measures, a little neglect, the O'Neill cylinder loses its environment. These poor people never get any luck. An enquiry shows nobody was really to blame, the company gets a token fine and you're home free.
Finding a planet for them is harder than you think
It's not actually as easy as you think to move planets. For example, we Earthers have evolved for billions of years to live on Earth in very particular conditions. If we were to move to another one, it wouldn't be good enough to simply go and find another oxygen planet with liquid water. The gravity would have to be similar, we would need similarly low levels of CO2, I'm sure there are many other gasses which would be deadly, or extremely uncomfortable to us if they were present even in trace amounts.
The climate, soil, etc. would have to be right to grow the food we eat, there would need to be plenty of water that is not contaminated with any chemicals or microbes that are dangerous or unhealthy for us. A similar magnetic field might also be required or highly recommended.
Now let's take your primitive race. You probably want to keep them as pure to their origins as possible - i.e. not using technology or any sort of body modification or drugs to help them adapt to their new habitat, going by the question it seems you don't even want them to know you exist.
It's probably easier to create an artificial habitat which can replicate their original conditions - i.e. those most suited to them - than it is to find a planet which matches their requirements closely enough.
Outside of space opera, FTL isn't going to happen, and interstellar travel is hard.
A "realistic" interstellar civilization is a mesh of systems connected together with lightspeed communication networks. They can still have ridiculous technology levels and even extend over galactic clusters without it being cheap to move large amounts of mass between solar systems.
They expand by sending star wisps -- tiny craft -- to new solar systems, which in turn build an entire civilization in that solar system (connected back to the old one via light speed communication). Uploading and downloading of life forms to digital representations is trivial, so even interstellar travel is possible; you just don't move your body.
When such a civilization finds a more primitive one that they want to save, building or even finding a new planet is going to be hard. I mean, they could copy them and upload them; but doing so isn't very polite to someone who doesn't consent. (uploading a copy and killing the original still kills the original).
Building a space habitat is much cheaper than building an entirely new planet.
The kind of problem that they couldn't block (cheaply) might be a supernova going off nearby. They build the habitat and shield it with the planet. When the supernova fades, the planet they used to inhabit would have its biosphere destroyed; the advanced civilization can then start reprinting a biosphere, while the rescued civilization lives in a orbital habitat.
After a thousand (or maybe million? I don't know how hard terraforming is) they can then move the rescued civilization back to the planet.
In a modern sense, this would be akin to identifying an endangered species next to an exploding volcano. Taking that species and keeping it alive in captivity during the explosion, then rebuilding the habitat as best we can, reintroducing the species to that habitat over time until it has recovered.
Finding a new habitat is harder (more expensive), and riskier, than trying to breed it in captivity. In the near future, we'd also try to backup the species with a record of its DNA and attempt to artificially produce copies of it (maybe in related species), but that would be a backup attempt.
Life basically is divided into growers (plants) and eaters (animals.) Growers have a very low energy budgets, almost certainly incapable of supporting civilization. Thus the race you're trying to save is an eater.
Eaters require an ecosystem to eat and an atmosphere that can be reacted with said ecosystem. Effectively, this means a world with an already-existing ecosystem and you probably have to seed it with a lot of lifeforms to make it metabolically compatible. Providing that is going to be a lot harder than just moving the species in question. No world without an existing ecosystem will have a suitable atmosphere--it's the product of life. (On Earth this would be oxygen, while I wouldn't say animal life must be oxygen based there has to be something and it takes energy to create.)
Thus moving them to another world is simply not viable. It has to be an engineered habitat.
It's not the only planet doomed to be destroyed by the same event.
If it's just one planet in the system, and they have a few years to plan, they could always commandeer any of the other, potentially uninhabited planets; especially if they're in the Goldilocks Zone - though if you're able to evacuate a whole planet, you could just plan to move another planet into the same orbit once the event requiring evacuation happens, you could just terraform that planet, and be generally good to go. Sure, the system's down a planet, but as long as you can keep it on the down-low, that seems reasonable.
As long as there are other planets in the system.
But what if we up the ante? After all, if there are no other planets in the system, then getting them to another galaxy would take a while, or finding a suitable planet nearby might take more work than is helpful. So you get them on an artificial habitat, and get the habitat out of there either with fair warning ahead of time of the event in question, or as the event begins and in a...bit of a rush to get them out of the system.
This might actually help with the mythological beliefs being used to cover your tracks - just, well, without staging anything if you're actually still leaving while the event is in progress. Depending on how similar their early civilization is to ours, you could get away with just letting them be able to see the event as they're escaping the actual event. Given some time, "I saw Pluto explode!" becomes "Pluto and Neptune got into a fight a while back" after a few years, and...that could get wrapped into a mythology with not a lot of trace for anyone who investigates well after the fact.
The Once and Future Home World:
Let us assume a cataclysm bad enough to wipe out life, but not actually blow up the planet.
Your native population will be destroyed by the cataclysm to come, but you either don't have the means or are ethically restrained from moving them away from their home system. Maybe they technically OWN their home world. Maybe every habitable planet is reserved for whatever intelligent species evolves there first. Maybe the mythology says they get to return to their home after the apocalypse. In a couple thousand years (a blink, really) their world will have recovered from catastrophe and be ready to be repopulated. So what to do?
You move them to a set of orbitals in their home system (maybe even over their home planet). They orbitally rendezvous periodically to allow genetic exchange, but are separate to prevent a single catastrophe from wiping them out. The sun looks the same, the plants and animals are preserved, and you don't have to drag them all very far. You use local materials (after all, the locals own the asteroids you build the orbitals out of) so you don't have to drag stuff from somewhere else. They can look down from the heavens and see when their planet begins to green - at that point, it should be ready to re-colonize.
And at that point, your responsibility to these people is done. You've saved them, left them with their own world, and even given them the landing ships to repopulate when the world is ready.
I mean seriously, you don't want to be on the hook taking care of them forever, do you?