Worm gears are used when large equipment reductions are needed. It’s quite common for worm gears to have reductions of 20:1, and also up to 300:1 or Planetary Gearboxes 
greater.
Many worm gears have a fascinating property that no other gear established has: the worm can certainly turn the gear, but 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 friction between the gear and the worm keeps the worm in place.
This feature is useful for machines such as conveyor systems, in which the locking feature can become a brake for the conveyor when the motor is not turning. An added very interesting usage of worm gears is normally in the Torsen differential, which is used on some high-performance vehicles.