I can't imagine what else it COULD be for. I've never seen a 110 plug on a vehicle with any other purpose.
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As a level 1 burglar, Bilbo got a pony when he accompanied the level 60 dwarves on the Smaug the Dragon raid. Those powerlevelers probably invited him solely so he could trigger fellowship attacks for them.
It is probably a block heater. If it goes to multiple locations, there's always the possibility that it does other stuff. Cars in the far north sometimes have block heaters as well as heating pads stuck directly to the transmission case and trickle chargers on the batteries.