I have books I've never opened since I read them that is just a marker like that. I have one copy I've never opened: I read it as an e-book but got the physical copy just to complete my set and have it on my shelf.

I also have books I've suddenly taken down to reference 20+ years after I read it, and where the text was not online, so I also like to have physical copies of books where the idea mattered because I know that some day I'll want to pass that idea on and will need a refresher.