what is this for?

So I'm on my second Jetta, picked up a 2006, mad cheap with only 22k on it... Have already started to fancy it up with blacked out windows and such...
well as I was exploring the new car I located a Solar Panel that has a 12v plug on it. Could someone shed some light on this for me? I have no Idea what purpose it serves...

Would it possibly be to charge my battery in case it died? hmmm......