For motor-just applications, the slower the electric motor speed, the bigger the motor needs to be. Large motors could be expensive and might not suit within the allowable envelope space of an application. Using a gearbox with a smaller sized motor enables the unit to use at lower speeds.
To be able to understand the importance of using a gearbox to diminish motor speed, think about this scenario. You possess a pastry speed gearbox conveyor that adds cherry filling as the pastries go by. Because this is usually a reasonably light load, you would not need the conveyor operating at 5000 rpm. If it was, there would be pastries and cherry filling just about everywhere!