power disconnect switch???

Cut off switch disconnects the battery completely without having to unhook the battery. Some items, like the carbon menoxide detector are always "on", as is the circuit board in your converter, in today's campers continuously draw power from the battery, which over a short time (a few days), will drain the battery completely. When not in use, you might not want everything drawing on the battery. By flipping the switch, you know everything is "off".

Also, if you park your camper (say, at home), and you leave your camper unlocked, and your kids like to push buttons, they won't be able to turn anything on, nor will they be able to open, or close the slides.