NOT DUMBLEDORE!Sketching is partly serious, but mainly the mind at play. It is not simple, but it is rewarding if you persist. It is also extremely useful in any activity requiring thought. I frequently sketch just to be creative. Much of the images are exploratory, and it is often easy to get discouraged when they don't immediately take a shape that one likes. That is why it is useful to do verbal sketches as well as visual. A verbal sketch requires the same tools of a pencil and paper.
Sketching, when it is serious, may be a valuable tool in creating thinking. The teacher Edward de Bono has many ideas about how one can use sketches to get your ideas into a form where they can be used. He developed the idea of thought hats where you assumed a role and took it to its logical or creative conclusion. Put on a type of hat and think in the way that the hat you put on would suggest that you thought. This idea can be expanded in many ways, and I am sure actors often have to assume thought hats when trying to get their characterization correct or interesting.
When I was a student at university, I would rarely take notes in the normal way. Much of my time was spend sketching logic maps of the lecture material as it was being delivered. I would use blank unlined note paper and my notes would look pretty disorganized to most people, but underlying them was the logic of the lecture. I rarely got less than an 'a' using this method and saved myself a lot of time trying to remember my notes. They just seemed to be more memorable.