For motor-only applications, the slower the motor speed, the bigger the motor must be. Large motors can be expensive and may 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 speed gearbox speeds.
To be able to understand the importance of using a gearbox to diminish motor speed, think about this situation. You possess a pastry conveyor that provides cherry filling as the pastries go by. Because this can be a fairly light load, you would not need the conveyor working at 5000 rpm. If it was, there would be pastries and cherry filling almost everywhere!