I think it has to do with feeling like a part of something. I have bought a couple of books here and there because I like supporting indie authors, and don’t mind reading various stages of drafts. One series I really really liked, and another was very unfinished but a good idea.

With tt authors you get the chance to give direct feedback and I think that makes people feel like they had direct influence on something they think is cool. Personally, I don’t give feedback unless specifically asked. That feels too much like unsolicited criticism for me, a non professional editor.