For speed gearbox motor-just applications, the slower the engine speed, the bigger the motor must be. Large motors can be expensive and might not fit within the allowable envelope space of a credit card applicatoin. Using a gearbox with a smaller sized motor enables the unit to operate at lower speeds.
In order to understand the need for using a gearbox to decrease motor speed, think about this scenario. You possess a pastry conveyor that provides cherry filling as the pastries pass. Because this is certainly a fairly light load, you would not need the conveyor operating at 5000 rpm. If it had been, there would be pastries and cherry filling everywhere!