I also do a form of sketchbook processing with writing, and yes, it is a form of brainstorming. It also helps me to tap into my imagination and concepts much better. For the most part, I write a page first of how I'm feeling (unplanned), and then draw something (no erasing) from that entry. There's more to it than that, but that's the gist of it in a nutshell. Not every page makes it into the painting stages, but some do. The concepts are always very raw, don't always make sense, and that is kinda the beauty of them. My advice would be not to be afraid to make anything that seems remotely simple, odd, silly, dumb, weird, wrong, or any other preconceived notion you might have. Just be free. Make what you feel. Not what you "see."