What is the primary purpose of grounding a raceway?

The ground wire is a low impedance return to the main service. In case of cable failure in the raceway and the cable goes to ground (short circuit), the ground wire conducts the high current directly back to the main service and the main breaker will trip shutting off the fault. Without the ground wire, the fault would have to return through the raceway or building structure in metal buildings which have mechanical connections which could be of high resistance.