Worm gears are used when huge equipment reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.
Many worm gears have a fascinating planetary gearbox property that no additional gear established has: the worm can simply turn the gear, but the gear cannot turn the worm. That is because the position on the worm is so shallow that when the apparatus tries to spin it, the friction between the gear and the worm retains the worm in place.
This feature pays to for machines such as for example conveyor systems, in which the locking feature can become a brake 
for the conveyor when the motor isn’t turning. One other very interesting use of worm gears is in the Torsen differential, which can be used on some high-performance vehicles.