From the things I have read about different ones have interacted with different places?
Like the "Nordic" ones were helping Nazi Germany but apparently other ones are involved with the US?
Is this being done to help us out or because they stand something to gain? Or are we the "planet of the children" like some have said and we are like their pets?
There are many species and factors so there is no single answer. But if you could put yourself in the mind of a highly advanced race/civilization (Something close to impossible for obvious reasons): vast in resources, technology, consciousness, etc why would you get involved in other civilizations?
But here are some factors for the more evolved species:
Some of Earth's resources are unique to our planet and useful to other civilizations. They are highly desired commodities.
Universal consciousness/"Galactic" collaboration growth.
Time is non-linear: Some species are not traveling through space but rather time. They know something about us that will make us completely rethink who we are, our place in the universe, where we come from, etc. And our time has the potential to change outcomes positively.
For a lesser evolved/hostile race:
Earth's resources
Colonization
Humans can be used for experimentation and scientific research
Humans are "farmed" and used for labor aka slavery
At the end of the day, it is much like the old "good and evil" fight on Earth. The good will always conquer. There will be bad times but the light will always prevail over darkness. The more evolved races want us to overcome our flaws, mishaps, and shortcomings because believe it or not (hard to when you see so much stupidity and "evil), humans have enormous potential.