The Art of Manliness discusses the tradition of note-keeping through the years in a blog post by Brett and Kate McKay.
"No matter what profession you find yourself in, the most essential function of the pocket notebook is to provide a place to capture the ideas that spring to mind throughout the day. You may get a business idea, an insight into something you or a loved one has been struggling with, or hear a quote you wish to record. Even though you feel sure in the moment that you’ll be able to remember these thoughts when you get home, every one of us has experienced the agony of realizing later that an idea is utterly gone from our minds and that no amount of mental gymnastics can bring it back…"
(Thanks to Anthony L. Stevens)