Other than an oxygen rich atmosphere and life in general it’s hard to see what an alien race would want with the earth…
Any civilization advanced enough for interstellar travel in reasonable time spans certainly can find any resources it needs all over the galaxy.
In spite of sci fi movies saying otherwise, water is abundant in the universe.. as are minerals etc…
Maybe they want to serve man?
Yeah DNA or "life" is about the only thing you can get from earth that you can not get in an easier to access form in space.
And if they have reached the starship stage they should be able to combine those things to make whatever they want.