Basically so you can control the lights from 3 or more locations. Usually only found in larger houses or custom builds. Depending on the layout and where they're located they are very useful.
That makes sense. I have 3 way switches in my house to control the upstairs hallway light from both the bottom and top of the stairs, and to control the breezeway lights from the house and detached garage. I imagine 4 way switches would be used in a similar situation.