Worm gears are used when huge equipment reductions are needed. It's quite common for worm gears to possess reductions of 20:1, and also up to 300:1 or greater.
Many worm gears have a fascinating property that no other gear established has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so gear box for greenhouse shallow that when the apparatus tries 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, where 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 certainly in the Torsen differential, which can be used on some high-performance vehicles.