A lot of my main characters write. Or know someone who writes. Or read a lot. The reason for this is it’s how I keep track of ideas.
Every writer knows that the moment you get words on a page, approximately 56,792 other ideas pop up. If I can’t use it in the story, and if it’s not enough to make me stop what I’m currently working on, it goes in as the characters wip or a book they read.