What is a three-way switch used for?

A three-way switch is specifically designed to control a single light fixture from two different locations. This is particularly useful in situations where you want to operate a light source from two separate points, such as at the top and bottom of a staircase or in a large room with multiple entryways. By using two three-way switches wired correctly, each switch can turn the light on or off regardless of the position of the other switch, providing convenience and accessibility.

The other choices involve functions that are not associated with three-way switches. For instance, controlling multiple light fixtures from one location typically requires either a single pole switch or a combination of multiple switches, not a three-way switch. Turning off the power supply to an area involves circuit breakers or main switches rather than three-way switches. Finally, modifying the voltage of a circuit is typically done using transformers and is unrelated to the operation of switches. Thus, the primary purpose of a three-way switch lies in its ability to allow control of a light from two different locations.