Well, I'm not an expert, but it's a bit like using a paint brush to spread the foundation around on your face. The advantage is that foundation doesn't settle into pores or lines because the individual bristles of the brush flex and bend to provide smoother coverage and thus more even color. There are specific brushes meant for foundation--I got mine from MAC, because while their brushes are expensive they are generally good quality. I'd assume you have to clean the brush every time, otherwise the foundation would cake up in it and then it wouldn't have that flexibility any more.