For me, Rhino is like a painter’s sketchbook.
One may argue if a painter even needs a sketchbook if his job is to paint on canvas. You can go straight to painting without sketching and trying your ideas, but it is very limiting. On the other hand, you can have amazing sketches, but most of the time you really need to go on the full-size canvas. Good luck with making canvas out of torn out pages from your sketchbook.
Also, you can find ideas from the sketchbook more open and interesting, but you are making living out of selling paintings, so you are once again in the store buying god damn paint.