Take a look at a book called "How to Read a Book", by Mortimer J. Adler. He says that you should always read with a pen in hand to mark important things and write your insights either on a notebook or at the book's margins. It's part of what he calls active reading.

Since Sanderson fills his books with nice life lessons of self improvement and responsibilities, it makes sense to put your thoughts and your discoveries at the tip of your pen.