I prefer to buy books to own. But books are expensive, so if a particular book feels like it's not something worth the money to keep, I just borrow it from the library instead. That's literally money saved for me. Yeah, you could argue that if the library wouldn't have been an option then maybe I wouldn't have bought the book at all, so no difference there, but it's still the difference between reading the book for free or not reading the book at all.