It can be model specific but at the very least it supplies some of if not all of the lighting, powers the thermostats/control boards for the furnace and refrig, powers the furnace forced air fan, provides the spark for the hot water heat unless it is a manual pilot light model, powers the water pump if needed, vent fans, radio and may also power certain safety monitors; LP detector etc. As you can see that is a lot of things and thus why the usual single 12v battery set up does not last long under the best of circumstances. Yes disconnect the battery when not in use and keep it charged fully.
