This is my guess:
Looks like it may have been added. Not original from the factory. If the water heater is behind that door I would guess the water heater has been changed to gas/electric. The switch may be to turn on the electric for the water heater.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.