I love the freedom of working in my sketchbooks!
Often, when I have some leftover paints after a painting session, I use them up in my sketchbook (hate to throwaway paint).
It's so strange that when there is no pressure of getting an end result and there is a sense of "it doesn't matter", some fantastic work can result.
Sometimes I also write down things I tried out. If I don't do that, there is a good chance that it will get lost very soon...