When might one use a three-way switch?

A three-way switch is specifically designed to allow control of a single light fixture from two different locations. This is particularly useful in scenarios such as staircases, hallways, or large rooms where you want the convenience of switching the same light on and off from different access points. Using two three-way switches in tandem enables you to control the lighting efficiently, enhancing both functionality and safety.

In contrast, the other options do not reflect the unique purpose of a three-way switch. Providing power to a single outlet typically requires standard switches or outlets, rather than a three-way configuration. Connecting multiple appliances can be accomplished through various means, such as using multi-outlet strips or series/parallel circuits, without the need for three-way switches. Ensuring a constant voltage supply is more relevant to circuit design and regulation rather than the specific control mechanism offered by three-way switches.