It is to fill the water tank when you don't have city water pressure. Say you are dry camping and you need to dump some water in the tank from a water jug. You use the gravity fill because there is a check valve in the city water inlet that won't open without water pressure.
You are lucky to have one, they stopped putting them in the new models. Another KS "plus".
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2014 Raptor RP300MP w/ Rear Patio Party Deck, Folding Side Ladder, 6 Point Level Up, Carlisle Radial Trail HDs
2004 2500HD D/A CC SB - TTT Mirrors, Prodigy BC, 18K Pullrite Superglide, NEW Fuel Injectors 11/2015 (ouch)