Worm gears are used when large equipment reductions are needed. It’s quite planetary gearbox common for worm gears to have got reductions of 20:1, and actually up to 300:1 or greater.
Many worm gears have an interesting property that no various other gear set has: the worm can easily 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 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 use of worm gears is usually in the Torsen differential, which can be used on some high-performance vehicles.