One of the first cars I bought had one on the floor. I bought it when I was 18 and knew bigger all about cars, and for weeks I was driving around tapping the button wondering what it was for. Was'nt until the guy in the car in front pulled over, waved me down, and informed me that if I continue flicking my lights at him, he would adjust one side of my face to the other!
Aside from being scared white, eureka! I now knew what that bloody switch did!
Cheers, Gareth