In my scifi book(s), the only reason aliens have to come to earth than say any other planet is to party. There's no reason to dominate or steal our resources. Go to any other system and you have infinite amounts of resources. In fact most star systems will have planets with life but not necessarily intelligent life.

So fundamentally it's just tourism. They come to Earth to drink beer and have fun. Though tourism is very rare because we have been so violent to previous tourists at roswell.

Obviously there are antagonists who do want to destroy humanity but generally speaking its about tourism.