I don't use mineral foundation (I use liquid) and I received a free (and really soft!) kabuki brush from someone, but I have no idea what to use it for.
Sheer blushes/bronzers/highlighters
Liquid foundation
Buffing out overdone blush or bronzer
Overall blending out of stuff on the face
Setting powder (I prefer a dabbing motion so as to not disturb whatever foundation I have underneath)