If it is momentary contact, it is likely a "boost" function for starting your engine with the help of the chassis battery. If so (and if it is working), you will hear a click when you press it as the isolation relay closes
If it is not momentary, there might be aftermarket fog lights or some such.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)