What is the point of Internal Water Tanks?

hitchglitch

I used to aspire to a caravan with an internal tank but now I wonder what the point is. After all, you still need to fill it using the Aquaroll and make two trips to the tap and you cannot travel with water in it. We keep a watering can with a wide filler next to the Aquaroll as a reserve tank. It holds 9 litres which is good enough for one shower.