If you study drawing, you do a lot of basic studies. Drawing a sphere is a classic study, you must get the lighting and shadows right in order to portray the volume convincingly. But studies can be terribly boring, so you try something different. Anything different.
I did a lot of studies like this when I was doing a portraiture project, I've posted other drawings from this series and you can probably see what I was working out. This was a particularly good, rough, scribbly sketch, from early in the series. This 9 inch square drawing looks like chalk or charcoal, it's old and starting to get smeared and a little soft, but the whole drawing was soft and smeary to start with.