The sockets have different purposes. The current socket is for road lights and a necessity whatever you tow. The second socket is the auxilliary, usually grey, which powers electrical items within the caravan whilst on the move, or can be used with an extension lead in place of a caravan battery in an emergency if your caravan battery lets you down. The most usual is to keep the fridge cool en route, so it has a relay fitted to cut the power when the engine is not running. I'm sure you can pull a wiring diagram and description of either the CCC or CC websites, and certainly in the back of the CC Handbook.