system
Given the following post, respond as if you're commenting on Reddit:
human
[General Sci-Fi] What are some realistic reasons aliens would want to conquer Earth?
gpt
I really like the concept behind the aliens in XCOM: Enemy Unknown.

Spoilers ahead if you care about that kind of thing.

So in XCOM you slowly learn through the course of the campaign that all the different kinds of aliens you are fighting are not one species, but several, that have been heavily modified through genetic engineering and mechanical augmentation. Eventually you discover a breed of psychically active aliens, and learn that all of the various species are connected through a psychic hive-mind tha is organized hierarchically, with the more psychically active species at the top.

By the time you get to the end of the campaign, you have appropriated a ton of the aliens' technology, and eventually create your own psychic super-soldiers, culminating in one super-powerful psychic; The Volunteer.

It turns out that all the aliens you had fought and defeated were failed experiments, attempts at creating a species that was physically and psychically 'perfect', something the alien leaders were never able to accomplish.

But you were.

The whole invasion was one in a series of conquests in which the "Ethereals" used warfare to force the evolution of a species to the point where they became psychically active. Mankind was the first such species to be "uplifted" successfully, by creating soldiers who were both physically strong and immensely powerful psychics.

Had the aliens had their way, humanity would hen have been assimilated into the collective and the aliens would have achieved 'perfection'. Another piece to the puzzle is that it is implied that the alien leaders were themselves uplifted by a powerful forerunner race, who eventually transcended physical existence, and the alien's pursuit of perfection is an attempt to follow in their footsteps, having been left behind.

**tldr;** Aliens use warfare to force humanity to evolve rapidly in preparation for assimilation into a psychic collective.