Keeping a sketchbook is essential for the working cartoonist or illustrator. It's a place to draw freely without the fear that someone will see your poor rendering of a bicycle. It's a place to return for reference about what a bicycle (sort of) looks like. It's a place to stow away underdeveloped or misguided ideas, in hopes they'll someday amount to something. It's a place to search madly for usable ideas when faced with a serious creative block. A visual artist who doesn't keep a sketchbook is like an author who can't find the time to read: they may be able to get the work done, but they won't have near as much fun doing it.