I use a small brush to get more control over the transition from light to dark. It enables me to take small steps, slowly changing the color from light to dark. It makes the transition more gradual.
"It seems to me that you could blend all your colors together without the need for stippling. QUESTION: Why all the stippling?"
The stippler is important to make the transitions as smooth as possible. Every time you add a slightly darker color you'll get a visible edge between the two shades. These edges are blurred by using the stippler. When you skip the stippler, and only use the badger hair fan brush, you'll get good results as well. With the stippler you'll get great results.
Do you have questions like this? I'd be happy to answer them if I can. Or you could download one of my tutorial video's at www.paintingskies.com/video