Same reason I buy other forms of entertainment
1) I think that the music (in this case) is worth the x amount of money I spent on it
2) It supports the artists (somewhat). Even if they already have millions and I enjoy their stuff, do they not deserve my money just because they already have a lot
3) Easier (for music). There are plenty of DRM free services that are easier than pirating. Hell, Amazon Cloud Player allows me to play my music from "anywhere" where I can get access to it as well as have it on all of my hard drives, etc. etc. *THAT* service is worth paying for music in and of itself alone for me.