In school we had to do small thumbnail sketches as prelim work for larger paintings. Consistently, I was "encouraged" to put a bit more detail in those drawings. I still don't.
Much like my handwriting, my thumbnails are "rough". However, my sketchbook is an extremely valuable tool to me.
I use it to work out formats and compositions, paintings for classes I teach, & to help me decide if I even want to do a larger painting.
I'm always in awe of artists who show their sketchbooks & they truly look like works of art.
That works for them. My crazy sketchbook works for me. :-)