The reason those generators run is because the cargo requires them, or ships would not be able to take anything refrigerated or frozen, removing a large part of very profitable cargo from them. If the power to those boxes fails for a long enough period of time the load is completely destroyed.

Ships use power for all sorts of things. Steering, ventilation fans, and water pumps just to name a few. Motor power is only a fraction of what a boat can do, and most boats connect a generator to the main shaft, because the power is more important than the motive force.