Worm gears are used when large equipment reductions are needed. It is common for worm gears to have reductions of 20:1, and actually up to 300:1 or greater.
Many worm gears have an interesting property that no additional gear set has: the worm can certainly turn the gear, but the gear cannot turn the worm. That is because the position on the worm is so shallow that when the gear attempts to spin it, the friction between the gear and the worm keeps the worm set up.
This feature is useful for machines such as conveyor planetary gearbox systems, in which the locking feature can act as a brake for the conveyor when the motor isn’t turning. An added very interesting usage of worm gears can be in the Torsen differential, which is used on some high-performance vehicles.