For motor-only applications, the slower the engine speed, the bigger the motor needs to be. Large motors could be expensive and might not match within the allowable envelope space of an application. Using 
a gearbox with a smaller sized motor allows the unit to use at lower speeds.
In order to understand the importance of using a gearbox to decrease motor speed, consider this scenario. You possess a pastry speed gearbox conveyor that adds cherry filling as the pastries go by. Because this is definitely a fairly light load, you would not want the conveyor running at 5000 rpm. If it was, there would be pastries and cherry filling just about everywhere!