Carbon brushes find wide application in electric machines such as motors, generators, etc. The function of a carbon brush is to transfer power from a stationary component to a rotating component or vice versa (See Article on Brushes).
Over time, brushes can get worn off due to age, friction, heat and arcing. Usually, the wear is all the brushes is uniform. However, there are situations where the wear on one brush is more than the others. Uneven wear can be caused by eccentricity in the commutator, uneven spring tensions,etc.Carbon brushes need to be replaced all at the same time. This ensures that the current is uniform. Mixing old and new brushes can cause uneven wear to the commutator and damage the motor.