Worm gears are used when large equipment reductions are needed. It’s 
quite common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.
Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, however the gear Super Power Lock cannot turn the worm. This is because the angle on the worm is indeed shallow that when the apparatus tries to spin it, the friction between your equipment and the worm keeps the worm set up.
This feature is useful for machines such as for example conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. One other very interesting usage of worm gears is definitely in the Torsen differential, which is utilized on some high-performance vehicles.