If we are ever invaded by aliens there are 3 primary motivations which probably apply to them just as well as to humans.

3rd Power: They want to dominate/enslave/exterminate us

2nd Sex: They want to have sex with us

1st Money: No they don't want our money. But they do want our gold. Few people outside of the astrophysics community understand just how rare gold really is in this universe. We are gold rich relative to just about every other location in the universe.

So if aliens invade and it's not clear what they want, they probably want our gold.