I used to be one of those artists who didn't think thumbnailing was important or I was just too lazy to do it. My work suffered from this attitude for years. Now however thumbnailing ideas, layouts, compositing and designs for just about everything I do is essential. If I don't I lose a valuable ability to work faster, harder and with a small visual guide to help me remember how I wanted a design, scene or illustration to look. I also always do my thumbnailing on physical paper, and not on the computer. There seems to be something about physically drawing them that makes the ideas stick more overall. It's the same reason that I write all my initial internal revision notes