Alien invasions of Earth have been a staple of science fiction storytelling since H.G. Wells. From then to today we have had stories of humanity versus an alien threat retold to us in many different ways. Many of us have almost always enjoyed these forays into human nature, but sometimes there is just something missing. There are occasions when the writers display a lack of thought into the science behind alien motivations in favour of a romp through science-fiction tropes, but to some of us—myself included—the science behind the whys and wherefores are just as important as the narrative on the whole.
One of the most common reasons we see in fiction for aliens invading the Earth is territory; the act of taking our home world for themselves, wiping out us pesky natives in the process. On the surface, this seems to be a logical reason for the launching of an invasion and a believable one. Let's put aside the idea of using science-fiction as an analogue to explore real issues for a moment, and we can see this concept start to unravel. How? It comes down simply to the level of technology these aliens can employ. They're putting together an invasion force, and constructing the machines and/or equipment needed to do so. They also must have ships that are capable of transporting an entire population insanely long distances. Clearly, if they can put the resources into something like this and have the tech available to sustain themselves on such a journey, why would they not look at the prospect of space habitats? Is there a particular reason that they're unable to colonise a slightly less suitable world such as we are planning to do on Mars.
It would be far less effort for them to construct their own colonies in space or on worlds where the natives won't be a problem.
Water is another example that is often used as a motive for invading aliens. Yes, it's true that Earth has a lot of water, and that it does indeed seem to be a necessity for life, so on the surface, it seems to be a very sensible thing for the aliens to want, even if we're just a pitstop to replenish their supplies. In truth, though, this is quite possibly the biggest bugbear that I have when it comes to 'reasons for invasion.'
The universe is absolutely packed with water. If we look at just the Sol System, Earth isn't even the home of the majority of that oh-so-precious H2O. The icy moons of Jupiter and Saturn have more than enough to placate anyone with the ability to harvest it, and even if it transpires that there is alien life hiding underneath their crusts, it's highly unlikely that it'll be in any sort of position to rise up and challenge an invading force, unlike us plucky humans.
A common defence of this motive is the liquid state of the Earths H2O, and it's a weak defence, to say the least. If the writer believes that an interstellar spacefaring civilisation is incapable of or unwilling to melt ice than they're probably the wrong writer for the job.
Slave labour. The aliens may want to take us as their slave workforce. They may arrive and demolish our military and capture what is left of the human race in order to enslave it, putting it to work on making the alien's next war fleet or reshaping the Earth into a new Vager IV. This certainly lends itself well to storytelling; a defeated human race building up a resistance movement, using guerrilla tactics and turning their own technology against them.
In reality, however, any civilisation that has reached a benchmark on the Kardashev Scale would have little to no need of a slave labour force. The chances are they would be a largely automated civilisation, and using robotics would be far more efficient than having to feed, clothe, and house biological workers. On the whole, it would just be illogical for such a civilisation to be using slaves. It is good storytelling but for the wrong reasons.
There may well be good reasons for aliens to attack Earth. Good for them, not so much for us. They may have ideological reasons for wanting to conquer, such as a warrior culture like Star Trek's Klingons, or Predator's Predator. They may have religious reasons—if religion survives this level of technological advancement. There may be no tactical or resource reasons for doing it at all; maybe they just do not view us as having a right to exist. Maybe to them hunting lesser species is a recreational activity or a generational right of passage. Perhaps they recognise our potential to become an advanced space-faring civilisation and would rather nip us in the bud before we become a problem; after all, we do not fear for our homes when a wasp gets in, but we don't let it stick around to build a nest.
Another common trope is that they will come for our resources, such as oil, or our fissionable resources such as uranium. Oil is something we ourselves are slowly breaking free from our dependency on, so surely it would be naive to think that an invading alien force, clearly superior in technological advancement, would have much use for it?
Almost every other resource on Earth can be found in abundance, across the universe. Except one thing. One tasty, little thing—and an alien race could very likely have a craving for it... Protein.
It's probably the most likely reason they would have to invade us. Not for our water, not for our land, not for our dubious abilities as slaves, but for our meat. Protine may be a very rare thing in the universe, and thus one of the most valuable things to possess. We're comforted by our knowledge that here on Earth we're top of the food chain, but that may not be true when we take into account the furthest reaches of space.
All we can do is hope that our future farm is free range.