You can get them in a variety of sizes, and the larger ones are very good for smoothing and shaping undulations, or muscles, or cloak/clothing folds.
The shop I go to has them individually just like brushes.
I've been building up a 'coat/skin' on some old chaos hounds I picked up on ebay, where you can see the ribs and internal organs, and I want to male them look like ordinary hounds, and I have used the larger shaper I have to 'blend' the greenstuff.