We could assume aliens are so different we will never understand them. But let's say for the sake of argument that they are more like us than we think. Let's say they are motivated by the same three goals we all have:

1 Sex

2 Power

3 Money

It's unlikely aliens want to have sex with us. And if they can fly here in powerful spaceships it's unlikely they want to dominate us (although they may take us all for slaves).

But money, now there's a key idea. Let's assume they want the same thing we want, a lot of money. No they don't want our paper, and they don't want our coins.

What they want is our gold. Few people understand how rare gold is in this universe. There is a lot of gold here on Earth, and probably a lot more in the asteroids. Gold is very rare and it's quite possible the aliens are coming here to take our gold.