Sketchbooks are a very important part for me because that it is where I practice and explore my ideas.
This is the place where I try to get away from the fear of not drawing realistic things and exaggerating specific moments. It is also a place to practice and research forms, shapes and proportions (to be exaggerated later).
One very useful method that I found good to make myself more comfortable with drawing cartoony and not quite realistic figures, scenes, etc. is to re-draw things that are already existing in that form. I think it helps with the experimentation and the journey to find something that is uniquely mine.
(I do believe that it is not stealing other artists work, because I am not using them in final pieces and they are in the sketchbook simply for practice and are tools to brake the creative barrier.)
In sketchbooks I try to explore everything and trying to get away from the perfection and do multiple thumbnails and drawings.