Sketches often capture more of the initial energy & enthusiasm than polished, finished drawings seen in publications. They also  can show, in one drawing, multiple, competing concepts in the creative struggle to find the best visual solution.
Sketchbooks can be, but are not meant to be full of finished drawings. They're for roughing out ideas. That' usually  a messy task as ideas are whirling around in the brain, trying to be the one(s) that makes it onto paper for real  consideration. Here are four pages from one of my sketch books.  I don't intend for them to be this way, but when ideas start to  flow, that's no time to clean up and finish anything.
I sketch realistically quite a bit. Some people are surprised, because I'm a cartoonist. I do it for two reasons, I just love to draw... and also the best way to improve cartooning skills is through observation that realistic drawing forces to occur. Cartooning is, after all, drawing what the world looks like, but distorting it with our particular style, and often simplifying it, but keeping the most important details that define each person, each place or thing.
But.....I love to draw! Period!