Very common in DC rotary motors - being as the motor rotates, it is impossible to have the current feed wires fixed to the connectors (such as through a soldered joint)
The carbomn brushes are made of graphite (a conductor) that are hard wired at one end, and spring (or otherwise) loaded to contact the rotary part of the motor.
This means that the contact is always maintained - and, one other advantage, is that graphite has a low co-efficient of friction, which means the armature (motor part) is not worn down. Graphite, being quite soft, will wear down however, but that means that the (relatively) easy-to-replace "brushes" need to be replaced and not the motor itself.