For motor-only applications, the slower the electric motor speed, the larger the motor must be. Large motors could be expensive and might not suit within the allowable envelope space of an application. Utilizing a gearbox with a smaller motor allows the unit to operate at lower speeds.
To be able to understand the need for using a gearbox to decrease motor speed, consider this scenario. You possess a pastry conveyor that provides cherry filling as the pastries pass. Because this can be a reasonably light load, speed gearbox you’ll not want the conveyor operating at 5000 rpm. If it was, there will be pastries and cherry filling almost everywhere!