For motor-only applications, the slower the engine speed, the larger the motor must be. Large motors can be expensive and might not suit within the allowable envelope space of a credit card applicatoin. Using a gearbox with a smaller motor speed gearbox enables the unit to use at lower speeds.
To be able to understand the importance of using a gearbox to decrease motor speed, consider this situation. You possess a pastry conveyor that adds cherry filling as the pastries pass. Because this is a reasonably light load, you’ll not want the conveyor operating at 5000 rpm. If it had been, there will be pastries and cherry filling just about everywhere!