I found this beautiful quote by Sarah Nelson. It's a great metaphor for thinking about how to use sketch books.

“A sketchbook is like soil. As things go into it they become one of two things: seeds or compost. The things that die in that collection become nutrients for future works, the things that thrive, grow beyond the page where it was born. It all has a place."  

Check out her work here  ​https://sarahnelson.art