The words written in the margin sometimes explain why the page was kept.

Annotation turns a text into a record of a reader. The printed words are the same in every copy; the pencil beside them belongs to one person on one day, arguing, agreeing, marking the line that arrived at exactly the right moment. That is usually the reason a particular copy survives a dozen clear-outs. What is being kept is not the book so much as the encounter with it.

Write in your books, if they are yours. Date the notes. Years later they will tell you who you were when you read it, which no clean copy ever can.