Third Conflict Ball From The Sun
Alien invasions (or humans invading other planets) invade Earth to get things that are more conveniently obtained in space.
Alien invasions (including those where humans are the invading aliens) often seem not to know how easy things are to get in space. They routinely invade Earth for its water (passing by entire comet belts made of ice), or for its metals (passing through an asteroid belt where the very densest materials are far more conveniently obtained). This is a specific form of science fictional Conflict Ball: the aliens need a reason to fight with humans, so plainly, they want our resources. Only, writers apparently didn't bother to check what resources a spacefaring civilization would have to fight over. Earth's water is relevant to whether aliens similar to us would want to invade us—because it exists as a solid, liquid, and gas on our planet, i.e. we're in the "Goldilocks zone" (not too hot, not too cold, but just right). Yet the aliens are remarkably seldom just interested in Lebensraum. Similarly, Earth has hydrocarbons that only form from organic life—yet somehow Hollywood has missed a lot of chances for a Green Aesop or "war for oil" analogies. Obviously happens because Sci-Fi Writers Have No Sense of Scale.
Hello, Unknown Troper. You'll need to get known to lend a hand here.