Worm gears are used when huge equipment reductions are needed. It is common for worm gears to possess reductions of 20:1, and even up to 300:1 or planetary gearbox 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 angle on the worm is indeed shallow that 
when the apparatus attempts to spin it, the friction between your equipment and the worm keeps the worm in place.
This feature pays to for machines such as conveyor systems, where the locking feature can act as a brake for the conveyor when the motor isn't turning. An added very interesting use of worm gears is definitely in the Torsen differential, which is used on some high-performance vehicles.