For motor-just applications, the slower the motor speed, the bigger the 
motor must be. Large speed gearbox motors could be expensive and may not suit within the allowable envelope space of an application. Using a gearbox with a smaller motor allows the unit to operate at lower speeds.
In order to understand the need for using a gearbox to decrease motor speed, consider this situation. You possess a pastry conveyor that provides cherry filling as the pastries pass. Because this is usually a fairly light load, you’ll not want the conveyor operating at 5000 rpm. If it was, there will be pastries and cherry filling all over the place!