Because we're dangerous.
The flipside of D. Hancock's theory, that we are uplifted to promote diversity (through providing alternate viewpoints), is that certain methods of seeming to do so will not only deny diversity, but prevent it - by making humans clients, possibly dependent on our benefactors, or possibly growing and furthering their culture, instead of ours.
Its a trap
So, if we are given technology, instead of developing it on our own, we are not likely to go back and discover alternate ways of doing things (potentially better ones) for quite some time, if ever. We will not understand the technology at first, we must take their word on it, and we will have to catch up to that level before we are comfortable modifying the tech or seriously questioning its principles - and even if some theory sounds unlikely, having devices that work will override that for most of us, so our progress might be further hindered.
The "uplifting" species might then be able to control what tech we get access to, what theories we learn "proof" for, and keep us with tech that they already know about, and that they can counter. It might keep us from developing the tech to be a threat to them - especially if to progress further than what they gave us requires theories or technology from the intermediate development period that we skipped, or takes a significant leap to get to the next stage. It will make us less of a threat.
It may make us dangerously dependent (the tech has some obvious weak-point or shutoff we don't know enough to counter, that they can trigger if we disagree). If an EMP or something disables electricity, we can survive a step backwards and rely more heavily on non-electric tech, and we already have some redundancies and backups in place because we know over reliance is dangerous - but a people (say, hunter-gatherer tribe) given electronics and become dependent on them without those intermediate steps will not know how to manage without. It might mean we abandon theories or tech that would be dangerous to our "uplifters", once we get boosted in a different direction - who wants to work on outdated, backwards tech when we have the shiny new stuff to play with (only, they are weak against chemical, and dumped physics in our lap so we never figured out how to defeat their "X").
I don't think this kind of setup would last forever, as humanity is curious and sneaky and always looking for an edge, but a) the aliens might not know that, and b) they might be content with a time-limited advantage, to gain some benefit until we can't be controlled.
Whose Empire is it, anyway
Alternately, the trap may be cultural, rather than military. It is possible that D. Spetz's answer is correct, that they plan to indoctrinate humans in ideas beneficial to them while using the tech as bait - but it is also possible that they are trying to assimilate us, instead of 'brainwash' us. Humans are clever and creative and pretty good at making things work. The aliens might think our potential too dangerous to be left alone - that once we develop our own tech, we will be as powerful, or more, than them - and will eventually have the potential to be a dominant civilization.
Maybe we don't have a tech advantage, but we might someday if we have another potentially dangerous advantage - because we're operating on different timescales (maybe faster), or we have psychological advantages, or we're really good at lateral thinking or loopholes, or whatever. Keep in mind that we're talking potential and eventually, but no reason that the uplifters can't plan ahead, right? But, if they grab us while we're still young, and relatively small (perhaps compared to multi-planetary civilizations) and they teach us what they know - we will uplift their civilization instead of our own, because we will be one civilization by then.
What discoveries we may make are right besides and working with their scientists and will advance their civilization, we will adopt their history and change their customs and laws (rather than make our own), our children will seek their tales, their adventures, and eventually forget our "backwards' culture, science, stories in favor of the greater opportunities they offer - and grow up integrated members of their civilization, with no chance to form their own (potentially greater one) independently. Like adopting a child to continue family traditions, they raise us up (genuinely) and in turn we see ourselves as part of their civilization even if "actually unrelated", and raise it up instead of competing for dominance.
(credit where it's due, idea comes from Poul Anderson's "Turning Point")
The experiment does what?
Alternately, we can go back and look at scientific purposes. They might be genuinely curious - like us teaching primates sign language, which is a very effective technology, just to see what they're capable of - and what we might learn about ourselves. Giving us tech is a way to look at themselves from a different perspective.
But, slightly more cynically, it can also make us a testing ground for unproven ideas. If there are experiments they want to see the results of that are dangerous or half-baked, they can give us the ideas, and watch our versions of experiments instead of risking their own people. How about testing social or psychological theories - especially if such experiments might be "unethical" if performed on their own people without informed consent (which might make some experiments tricky). But, humans might not have the same status - or maybe so can be "offered" information that leads to the situations, because 'observing' isn't the same thing as deliberately setting up such an experiment, of course not! If they aren't sure about adopting some idea, they can play down their concerns, offer the base information, and see how crazy it drives us, and what loopholes they didn't think of, and how society might change if they adopted the idea.
Obviously, such experiments won't be absolutely relevant to them and their society, any more than mouse or monkey testing is absolutely relevant to us - but it can give ideas of what to look for and how careful to be when they are working with those they don't have to be quite so careful with. "uplifting" humans to their level would be one way of eliminating variables (access to vaguely similar tech, exposure to vaguely similar ideas) or baiting the trap of proven tech, to get us to accept and experiment with unproven tech and radical ideas.
Actual tech-trade
And now for something completely different!
The reverse of the situation in the "Its a Trap" section, where we mightn't be able to cope if tech was withdrawn, and we didn't have intermediate steps to fall back on... maybe that's what the uplifters are looking for, having lost their own ability to step backward. There's an idea now, to look at cultures with less advanced tech, and re-evaluate their 'traditional wisdom' for ideas that might have been missed the first time around (due to cultural superiority or superstition). There's an idea for looking at historical methods and 'traditional' technologies for survival or reenacting or just so they don't get lost, if we ever need them. And we have enough cultures, with different tech levels and yet similar enough for understanding, to mostly bridge the gap - though there are some things still lost to us in history, sadly, from when we weren't as careful about preservation.
So, I'm imagining the uplifiting culture has missing gaps - from "low tech" solutions, to survival skills, to all the things their culture might not have valued at the time in favor of more 'advanced' ideas and tech. And maybe they are unified enough, or advanced enough, they don't have other cultures to borrow from to fill the gaps or they just reached a place they can't step back from. But, humanity might have the missing gaps - or at least make some headway on filling them in. Maybe there's an actual reason (they actually need to move in or colonize areas with little tech, and don't know how to survive there), maybe they have a problem that needs a lower=tech solution that they've forgotten how to figure out, maybe it's just the equivalent of hoarding knowledge by survivalists and fans of post-apocalyptic fiction that tells them they should know how, just in case.
So, they can trade the shiny tech, for our current, primitive methods. They don't even have to admit the problem, just ask to observe our culture (for comparison purposes, of course), and maybe access to historical records or entertainment (like reenacting, it will look harmless). They're willing to hand over shiny tech because its what they have on hand to trade with, or because it's pocket change to them (shiny bead equivalent), and because they don't care about what it might do to our civilization, as long as they get what they need for theirs (as higher tech cultures have traditionally traded with low-tech ones, really, "non-interference" is storybook nonsense).
Unlike most of my other ideas, this would be genuinely, or fairly genuinely, a trade for mutual benefit. They can get the things they need - "primitive" but useful tech, ideas, survival knowledge, knowledge about how to adapt or innovate or progress with limited resources. We get tech, and support, and with access to ideas and trade eventually hit a roughly equal level of technology.
