They're cheap, and unlike Kindle books, the more popular they are the cheaper they are. If I know there's a good chance it will only cost me a few pounds to get a copy, I'll be more likely to ask for recommendations and keep track of things I want to read.
If I really like a book I've got second-hand, I can display it in my house, or better yet give it to a friend. If I don't like it, maybe I know somebody who will - or I can donate it to charity.
Thanks to this realisation, I went from reading 8 books in 2017, to 16 books in 2018.
Having a book on hand also fits right in to a digital detox, though it's even healthier as a regular part of my mental diet, rather than a last resort.