Sketchbooks for me are arguably my greatest tool in terms of games development. I've carried notepads with me for at least 10 years and it's helped me develop so much.
I recently sat down for an afternoon to go through my old sketchbooks and it's amazing how many gems I'd written down and then forgotten about, many untouched not due to the value of the idea, but due to my lack of programming knowledge at the time they were written. Noting these ideas down preserve them for a time where your skill reaches the new heights required to bring them to life, a bit like cryogenic suspension in a weird kind of way...
Anyway, I wrote an article about the benefits of carrying a sketchbook a while back, feel free to give it a read:
http://fireside.gamejolt.com/post/take-note-why-having-a-sketchbook-shouldn-t-be-overlooked-ryuy4mwe