I would never deface a book by writing in it (unless of course it is a notebook), but it is common when someone buys a book as a gift that they write some message inside. Then when you buy a used book you get a tiny glimpse into some stranger’s life.
For example I’ve seen messages like ‘Darling Debbi, love you forever xxx’. Which is sweet, except, now I have Debbi’s book that I found in a charity shop, suggesting that maybe the love didn’t last forever. Maybe they had a difficult break up and Debbi decided she had to get rid of everything that reminded her of them. Maybe some tragic deaths were involved. Maybe Debbi just didn’t like the book and hoped she could give it away without her partner noticing – if they ask, she’ll just say she loved reading it so much that she was reading it outside and must have put it down somewhere and forgot to put it back in her bag. Now Debbi has five of the same book that she doesn’t know how to get rid of.