The serrations do help the brushes seat to the comm faster, but I think they also make the motor run better. One theory I've heard is that the serrations reduce the friction between the brush and the comm, another is that electricity flows along the skin and edges of a conductor and the serrations provide more edges for a more efficient current path.
I'm not sure why they run better with the serrations, and to be honest I'm not really 100% certain that they do indeed run better with the serrations. But they seem to...