A commonplace book is a tool to catalog ideas, quotes, factoids, and other sources of inspiration. The act of writing them down helps me minimize the number of times that I remember stumbling across an insight or thinking of a good idea only to have forgotten what it was. Unlike a diary that's written chronologically, I try to cluster entries by concept so that they're easier to find in the future. Typically, I'll scribble a doodle next to the ideas on the opposing page to jog my memory when I go to find them again.
This low tech hack is surprisingly effective. While many people use tools like Evernote for this, I've found that the act of physically creating the commonplace book does more to help me connect the dots than being able to exhaustively index and search its contents.