For motor-only applications, the slower the motor speed, the bigger the motor must be. Large motors can be expensive and may not fit within the speed gearbox allowable envelope space of an application. Utilizing a gearbox with a smaller sized motor enables the unit to use at lower speeds.
In order to understand the need for using a gearbox to decrease motor speed, consider this situation. You have a pastry conveyor that adds cherry filling as the pastries go by. 
Because this is definitely a reasonably light load, you’ll not need the conveyor working at 5000 rpm. If it had been, there will be pastries and cherry filling everywhere!