Worm gears are used when large gear reductions are needed. It is common for worm gears to have got reductions of 20:1, and even up to 300:1 or greater.
Many worm gears have a fascinating property that no additional gear set has: the worm can simply turn the gear, however the gear cannot turn the worm. This is because the position on the worm is so shallow that when the apparatus attempts to spin it, the planetary gearbox friction between the equipment and the worm holds the worm set up.
This feature is useful for machines such as conveyor systems, where the locking feature can act as a brake for the conveyor when the motor isn’t turning. One other very interesting utilization of worm gears can be in the Torsen differential, which can be used on some high-performance cars and trucks.