Agree'd with Bricekin. Probably had a 7inch DVD capable head unit, and wired the switch to allow it to play while driving. (only for passengers of course!)
Either that or possibly used it as a power switch for offroad driving lamps, or even an amplifier power switch. Ya never know the kinda stuff DIY'ers will do. I used a switch for my offroad lights, though I must say its an odd place to put it. Let us know if you figure it out!