3 Way Switch Controlling 2 Lights. Three-way wall switches control a light fixture from two locations in a room. Another type is the four-way switch, which is used in conjunction with two three-way switches to control lighting from more than two locations.
In building wiring, multiway switching is the interconnection of two or more electrical switches to control an electrical load (often, but not always, lighting) from more than one location. Your answer is like saying why do we call a Hammer a Hammer or a Nail a Nail. I have a light at the top of the stairs and a light at the bottom of the stairs.
There are also four-way, five-way, and a gazillion-way circuits!
In the beginning they were called single pole, double throw.
Three-way switches & light fixture with duplex receptacle. This guide will take you through the steps of replacing those aging three-way light switches with updated wall controls. Asking because I've installed three sets of these in my home (newly built) and two of the sets are functioning weird.