Caravan Leisure Battery, the Basics Thereof?

I am going to declare my ignorance here and you are welcome to laugh at me.  But once you've finished laughing, please educate me!

What does the caravan's battery actually do?  It's something I've never given any thought to.

We don't have a motor mover and we don't switch on any gadgets till we get set up and plugged in to the on-site EHU.  Then, presumably, everything is powered by the electricity from the EHU.  So what does the caravan battery do?