For motor-only applications, the slower the motor speed, the larger the motor needs to be. Large motors can be expensive and may not match within the allowable envelope space of an application. Utilizing a gearbox with a smaller sized motor enables the unit to operate at lower speeds.
In order to understand the importance of using a gearbox to decrease motor speed, consider this situation. You have a pastry conveyor that adds cherry speed gearbox filling as the pastries go by. Because this is a reasonably light load, you’ll not need the conveyor operating at 5000 rpm. If it had been, there would be pastries and cherry filling all over the place!