For motor-just applications, the slower the engine speed, the speed gearbox larger the motor must be. Large motors can be expensive and may not fit within the allowable envelope space of an application. Utilizing a gearbox with a smaller sized motor allows the unit to operate at lower speeds.
To be able to understand the need for using a gearbox to decrease motor speed, consider this scenario. You possess a pastry conveyor that adds cherry filling as the pastries pass. Because this 
can be a fairly light load, you’ll not need the conveyor operating at 5000 rpm. If it had been, there would be pastries and cherry filling almost everywhere!