A common sight in science fiction is that aliens pretend to be human (For example in Third Rock from the Sun). Obviously if the aliens are advanced enough to disguise themselves as another species, there are much easier, simpler and less expensive methods to destroy humanity, so why else would an advanced alien civilization waste time, energy and resources to disguise themselves as humans? What possible scientific, cultural or commercial use could such an expensive procedure have?
Aliens are alien, without a common background their motives may be incomprehensible.
If we assume that these aliens are Hollywood aliens (ie humans with odd-looking rubber bits stuck on their faces) then there are all of the reasons that humans from one culture spend time in another culture and try and learn to fit in:
- Anthropology; studying how a different culture has developed to better understand the developmental processes that influenced your own. Iain M. Banks wrote Inversions (two anthropologists from the Culture investigating a Renaissance-era world) and The State of the Art (a Culture Contact team on Earth ca 1970) in this vein.
- Tourism; experiencing a culture that is vastly different from your previous experiences.
- Reality TV; Bear Grylls.
- Method Acting; preparing for a role in their own entertainment channels that involves playing the part of a member of an Information Age society.
- Challenge; because they can.
- Hermitage; there are people here on Earth who prefer to step away from the rat race and all the newfangled gadgets and live as their ancestors did, but usually not all the way back to stone tools. Selecting a developing world and fitting in gives them access to a level of technology between their home culture and being trapped on a desert island.
- Camouflage; they are being sought by members of their home culture and it's easier to hide and survive amongst us than in their own milieu. There are a lot of examples of this in fiction already: For Richer or Poorer, Men in Black II and it's analogous to fleeing the law and retiring to a country without an extradition treaty.
- Crash Landing, or a failure that prevents them from going home; if they came on an FTL ship that doesn't have FTL communications they may be waiting a long time for spare parts or updated navigational maps, or if they brought Gilligan who proceeded to ruin all of their plans to return home, or they may have arrived on a slower-than-light colony ship and would prefer to stay here and fit in amongst us rather than try and find another prospective colony.
- Uplift; meddling to steer us onto the "right" path of development and prevent us from immolating ourselves (or vice versa if your species doesn't like the competition).
Probably the most likely reason is plain old curiosity. We've had people go and live among apes, wolves, or other animals in order to study their habits, and if we had the technology to make ourselves indistinguishable to them we would be able to learn a lot more.
We don't really need a reason for such ventures, it's just human nature to want to understand things. This is the same drive that has driven much of our interest in space, so it stands to reason that at least some aliens who managed to achieve space travel have the same drive to understand the unknown as we do. And unless the disguise technology is prohibitively expensive, it isn't exactly a species-wide venture, just the private activity of a few alien Jane Goodalls.
They're anthropologists (literally!), and they don't want subjects who know they're being observed to bias their findings. Their goal isn't to destroy humanity, but to understand how humans work: discovering how a different species and society functions can provide a useful contrast with their own culture (which is why anthropologists study societies around our planet today). This might also be of concrete benefit to these aliens by understanding how humans might be of benefit to them, or -- as you suggest -- discover anything of value in humanity worth stealing before annihilating us with their superior weaponry.
I hate to be the only capitalist in this crowd, but the answer is obvious...
They're doing market research!
It is against intergalactic law to harm an emerging species until they survive the atomic age without outside interference. There are no rules however, against covertly visiting the planet to find out what its strengths and weaknesses are. What unsanctioned technologies do they need most? What resources, art, literature and other valuables do they have which might be trade worthy in the galactic market?
The alien infiltrators are entrepeneurs, trying to get a jump on a new emerging market.
I am a bit of a tech junkie. For this reason I, in my household, have more advance technology than some small organizations in third world countries possess.
This being said the idea of me conquering and overthrow the vast majority of such organizations is ludicrous. I lack the skills, the supplies, the logistics, and the manpower, and frankly my technology isn't that useful for war making.
Perhaps these theoretical aliens face similar problems.
They want to covertly control humanity
Who are they?
This is a race of invasive aliens (hereafter called Exploiters) who travel through the galaxy, with the wish to enslave all other intelligent species. Humans are seen as a desirable target because there are so many of them, and they are seen as naive, yet intelligent and dexterous enough to be useful. The Exploiters feel they are the natural rulers of the galaxy, and that all other intelligent species should be under their control.
How do they take control?
First, they use their advanced stealth technology to caputure the US president. They absorb his/her DNA, appearance, voice, memories, etc, and then murder him/her. They immediately replace him/her with one of their Exploiter minions, looking and sounding exactly like him/her. They then proceed to do the same thing to other powerful people, politicians, media personalities, powerful industrialists, etc. All over the world. The more people they capture, the easier the task becomes, because they can use their preexisting minions as aides in capturing more people and covering stuff up.
Why do they want control?
Once they control the governments, legislative bodies and mass media in the largest countries on Earth, they can easily wield that influence to use humans as their puppet race. They can introduce new technology to humans, for the Exploiters' own benefit. They can even reveal their own existence to humanity, pretending to be a friendly race. In reality, the Exploiters are doing it for their own selfish motives. Humans will be cheap labour in their factories (humans believe the industries are owned by humans.) They will be soldiers in a vast galactic fleet (humans believe it is an International Human Space Navy, controlled by humans.) The Exploiters can get humans involved in all kinds of interstellar wars. For example, the Exploiters are invading Planet X somewhere else in the galaxy. It will be a fierce battle, and they don't want to send their own people to die. So they stage a fake alien invasion on Earth, using the controlled media, scientists, NASA, etc., to blame it on Planet X. Acting like a friend to humans, the Exploiters then give humans enough technology to "defeat" the invasion forces. Earth's population now sees the Exploiters as their best friends. The media, NASA, governments, the ambassadors and scientific envoys of the Exploiters, etc. all convince humanity that they need to launch a preemptive strike on Planet X, before they attack again. The Exploiters offer to give humanity the technology to carry out the attack. All humanity will have to do is to act as workers in the defense industry and as soldiers in the war against Planet X. Humanity feels thankful. They feel they are being saved from being exterminated by Planet X. In reality, they are unknowingly fighting and dying as puppets for the Exploiters.
Doing this, the Exploiters have gained billions of servants, doing work they themselves feel too exalted to engage in. Humans can die on the galactic battlefields for them, while the Exploiters are the covert commanders. All the while, the Exploiters always make sure that the powerful people in human society are always the (disguised) Exploiters themselves. This is prioritized so highly that there is never any risk that humans will figure out what's going on, let alone take control. The Exploiters could still maintain an illusion of democracy. Using the media, the Exploiters will sway public opinion to favor candidates who are actually Exploiter minions. If any real human candidate happens to become popular, then he/she can simply be killed and replaced by a minion. Or, a fake sex scandal or the like could be engineered to take out said individual. Such things could also be used to blackmail or otherwise control any real humans with some amount of power. A few humans may occasionally suspect something, but they are easily dealt with. They can be killed, marginalized, ridiculed, put in mental asylums, etc. They will be a tiny fringe.
The Exploiters will never reveal to anyone the fact that they have the ability to emulate other species. This will be one of their most tightly guarded secrets.
Thus, the Exploiters don't want to kill humanity. They want them as servants and soldiers, to work and die for them.
Add some hope to the world
Oh well, this sounds like quite a depressing world! To cheer people up, the protagonists in the world could be an inter-species alliance of individuals (including humans) who know about and are fighting the Exploiters. They use covert tactics to expose the workings of the Exploiters. Can they find a way to reveal to humanity and other controlled races what is being done to them? For example, what if they find a way to sabotage the emulation technology? Suddenly, thousands of minions will be instantly revealed for what they truly are! But has the Alliance itself been infiltrated by Exploiters? And can they achieve success without being stopped?
They'll pose as us in order to study what love means to humans.
Presumably, if they've already studied us enough to pose as us, they'd already know much about our culture. They may even have heard rumors about the concept of "love". However, if the aliens themselves have no concept of "love", they might send aliens posing as us in order to learn more about this abstract concept.
If they are trying to destroy the planet, maybe they are outnumbered. Even with their mothership, fighting a whole planet of 7 billion might be like fighting zombies - they just keep coming. They might only have 1 ship, or a limited crew. For example, one of the USA's aircraft carriers might do a lot of damage, but cannot invade an entire country.
Because the 'expensive procedure' of super-advanced aliens disguising themselves as human beings isn't expensive for super-advanced aliens and their super-advanced technology. There is a straight forward reason why they'd do this, they will have adopted the morphology and biology of successfully adapted species to Earth's environment. Beats space-suits or environmental hazard gear and you can blend in with the primitive natives.
Now unless I miss my guess being disguised as humans isn't a way to destroy humanity. Human disguises as the prelude to global genocide & world conquest might be a cunning strategy. Good old fashioned espionage and sabotage, perhaps. Recruiting quislings, agents of influence, and bringing our social, political and cultural elites onboard to make the take-over easier. Colonial empires often used sections of the societies they colonised against those self-same societies.
Anthropologists during our colonial period studied the natives to help make colonisation run smoothly. Alien anthropologists might play the same role.
On the other hand, there might be a galactic peacekeeping organisation, something like the United Nations but with more super-advanced weapons and infinitely greater caapcity and tendency to conduct military in the name of galactic peace, that has this unfortunate tendency to exception to super-advanced alien species invading planets full of non-starfaring primitive sapient species. In which case, covert invasions would be the way to go. Outwardly our totally subverted and conquered planet would appear to run by members of H. sapiens sapiens as if nothing had changed but in reality it is oppressed under concealed iron tentacle of an imperialistic super-advanced alien species whose true form is so horrible and ghastly it would scare the corn flakes out of Cthulhu and its cute little friends.
They are evaluating humanity as a species - forming relationships, monitoring trends, and living among us - for entry into a larger galactic confederation.
They can't truly understand us and come to a credible judgement without first allowing us to expose ourselves to them, making ourselves vulnerable via bonds of friendship and trust.
If they manage to assume a position of influence on our world, this also potentially places them into a position to help guide us away from inadvertently disastrous paths, such as nuclear war, destructive biological research, or murderous artificial intelligence.
Think of it like an intergalactic "Big Brother / Big Sister" program. On a planetary scale.
They don't want to alter our behavior
Why not? Politics.
Suppose the aliens' Great Council of Killing Species has been performing its titular function for millions of years. At some point, assuming these aliens have ethical concepts, there will no doubt be a Great Council of Judging Species, which goes in advance and observes species in question to determine whether they need to die.
In time, after discovering that every species that they observe seems to have behavior patterns that can be summarized as "running around like crazy, shoving each other, extreme self-concern, screaming and general panic," they may come to the realization that the GCoJS's presence is quite possibly the reason for said behavior patterns, and if so, a Great Council of Minimizing Observer Effect will no doubt arise.
The GCoMOE may then regulate that any contact the GCoJS has with a species for purposes of judgement must be made is as inconspicuous a way as possible, including disguising their own interplanetary vehicles as the target species' own vehicles and their bodies as the target species'.
The most basic reason that anyone does anything...
(or lust or whatever)
The aliens want to get with the humans, whether it is out of pure lust or a desire to propagate a sort of hybrid species for evolutionary purposes. However, they know that humans are a bunch of racist bigots, and as such, (outside a certain minority of niche fetishists) they would never accept a tentacled, prehensile-organed slimy ball of goo as a romantic partner.
We go through a lot to understand all we can ancient cultures--we go to great expense to examine, learn about and preserve them just because they are interesting to us. I think that's reason enough.
The reason they are in disguise is the question, but I think it's not all that tough. Every less advanced civilization we've encountered has been--well pretty much destroyed just by contact with a more advanced culture. Even knowing about the more advanced culture destroys a lot of the motivation to retain their own separate individual culture. The people often continue but their original cultures are all but completely lost.
We now try to minimize contact with the few remaining "Uncivilized" tribes we know of because of this. Seems like aliens are better at this than we are since most of us still don't even know they exist.
Just because they're from another planet doesn't mean that they're not just like us. In fact, for there to be any form of communication at all, there has to be a common frame of reference, and if they're disguised as us then there has to be a lot of common frames of reference, therefore, they probably are just like us. In which case, they're here for the sex.
Ok, back to the real question then. Why would super aliens mix with us? Here are some suggestions: - too much inbreed in their own dna - boredom with being a super alien (winning WordFeud all of the time) - looking in the mirror and being jealous of Dolly Parton's tits - they spoiled their own planet and believe that maybe, just maybe, we can still save our own - on the above they just come and have fun with us for doing exactly what they did themselves
Plenty of reasons, but if I were an advanced alien I would travel to Alpha Centauri with laser propulsed nanobots just out of curiousity. Maybe Elon Musk and Stephen Hawkins are advanced aliens too, they are planning exactly that.
IMMEDIATE GOAL: Assessment of humans on planet Terra (Earth)
- Sub-Goal #1: Where is Terran technology in contrast to ET tech?
- Sub-Goal #2: Where is Terran culture in contrast to ET culture?
- Sub-Goal #3: What is the likely ETA for Terrans to enter Galactic civilization?
- Sub-Goal #4: What risks do Terrans pose to Galactic civilization?
ULTIMATE GOAL: Intervention
- Sub-Goal #1: Based on the various assessments above determine what if any interventions might be necessary to prevent significant harm to the Galactic civilization.
- Sub-Goal #2: BY means of infiltration (disguise) sabotage or modify Terran science and/or culture to lower risks to the Galactic civilization.
IN A NUTSHELL: Galactic Babysitting
It's a kink.
Dressing up as alien species, let alone actually making out with them, is abhorrent to the majority and/or ruling class on their home planet(s). So, they have to sneak off to alien worlds to get their fix.
But then it gets kinkier.
If it's really abhorrent to their race/government, they will feel compelled to stop them, and that means sending the police. However, they probably also have rules about non-interference-- specifically, not revealing themselves to species which have not discovered interstellar travel-- which means the cops have to dress up, too... which means your safest option for satisfying your kink is to join the police.
I think this is exactly where an author like Samuel R. Delaney would have taken this. (Read his incomprehensibly underrated masterwork, "Stars in My Pocket Like Grains of Sand", if you're curious.)