Worm gearboxes are usually for large gear reductions usage. It is common fact that worm gears have ratio 20:1, even up to 300:1 or much greater.
Most worm reducers have an interesting feature which no other reducer device has: worm shaft could turn the worm gear easily, but the worm gear couldn’t make the worm shaft move. The reason is that the angle on the worm shaft is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.
This advantage is very useful for conveyor machine, in which the locking feature can be as a brake for the conveyor when the motor is not turning.