This acts as a note keeper, bookmark and sort of time capsule for me. When I finish a book, i’ll always leave it in there. I don’t know why… but I just like the idea of random people (and my kids!) finding these random photos and notes in my books long after i’m gone! The photo in the book also links it to that period of my life.
I found a strange polaroid in a 2nd hand book when I was a little kid, then some years back I used a photo of my own as a bookmark, the memory bubbled back up, and here I am now. Wife and I kept saying we don’t print enough hard copies of photos, so this helps fix that a little bit as well - you could say I now print too many!