Multiple brushes on a commutator
Multiple brushes are also needed to handle the current in the armature circuit.  High power motors need multiple brushes just to handle the current and keep heating down to a reasonable level.  

It isn't just motors either, here on some high power slip rings, the slip ring is 18" diameter of brass, and each one has 168 2" carbon/metal composite brushes on them.  Neither carbon nor carbon/metal composites conduct anywhere nearly as well as copper.