The Shapes_Fill brush is a very strange tool at first sight! I have wondered why Krita even included it, but to my amazement this is an extremely helpful tool for brainstorming and quick thumbnailing. Check out Alchemy if you haven’t heard of it before. I haven’t used Alchemy before and this is my first attempt at it in Krita.
EDIT: I went and tried it out on Affinity Designer, with “Frankentoon – SciFi Starter Pack” brush set. I wish Krita can do something like this, where the brush can stretch out the texture/brush tip as you draw it.