It disconnects the univolt converter and the DC circuit panel from the coach batteries. This is useful for storage, or if you have a 12v
drain somewhere, to keep from killing your battery when not plugged into shore power. I also disconnect it when working on the converter or any 12v
wiring to avoid accidental grounding of the hot wires. The newer converters are able to power 12v
loads up to rated amperage without the batteries connected, so it can be off/open when on shore power.
The old univolts are known for killing batteries by overcharging them. Best thing to do is to replace it with one that has a "smart" charging profile.