For motor-just applications, the slower the motor speed, the larger the motor must be. Large motors could be expensive and might 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 importance of 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 usually a fairly light load, you would not want the conveyor operating at 5000 rpm. If it was, there will be pastries and cherry filling everywhere!