Like a few other people said, I'll buy books that I might want to read repeatedly. But also, if I think a book was particularly well written, I'll buy it as a way of supporting the author and showing that I'm interested in more of that type of content.

I'm sure disappointingly little of the money I spend actually makes it to the author, but buying it seems like a logical first step.

Of course this is limited by my income and how much space I have to store books, if it's a physical book. (Currently all my bookshelves are overflowing so... the competition is tough lol)