Always done this since I was a kid. It’s great if you have a short economy of space on your desk. Even now that I’ve moved to Uni, I still often find myself using my bed as a desk while I sit on the floor.
Doesn’t do my arthritis any good though.
I sit on the floor to draw, resting my sketchbook on a large hard-back book.
I would sit on the floor but my back is just the worst. Then again, it hurts to sit in a chair for too long as well. I’m considering getting a stand-up desk, as I’ve heard from some illustrators that they help