Worm gears are used when large gear reductions are needed. It’s quite 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 established has: the worm can Torque Arm simply turn the gear, but the gear cannot turn the worm. This is because the angle 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 pays to for machines such as conveyor 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 is in the Torsen differential, which is utilized on some high-performance vehicles.