For motor-only applications, the slower the electric motor speed, the larger the motor must be. Large motors can be expensive and may not fit 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 need for using a gearbox to diminish motor speed, consider this scenario. You possess a pastry speed gearbox conveyor that provides cherry filling as the pastries pass. Because this is a fairly light load, you’ll not need the conveyor running at 5000 rpm. If it had been, there will be pastries and cherry filling all over the place!