I buy books that I have already read if I would like to: have one around, reference, reread, recommend/loan/give, or to start conversations.
If a book was important to me intellectually or emotionally, just having a copy around and just noticing it occasionally reminds myself of whatever was important about it, and that can be valuable to me.
I posted this comment on a similar topic a while ago, for context it was replying to someone who wanted to pick 2 LeGuin novels to read to essentially get a survey of her work. I’ve liked her standalone novels as well, but I see them get less discussion generally. I think her work that I see referenced most often is the short story The Ones Who Walk Away from Omelas.