A lot of the furnaces in the Midwest have a similar device to the picture, but with a switch instead of a receptacle and the furnace itself is hardwired. A service tech told me that's so he can unscrew the fuse for LOTO instead of just hoping that the homeowner doesn't turn the c/b back on.
#210075 - 05/25/1302:30 PMRe: Overload Protection for Air Handlers
I've installed a few of those, as a way to give the equipment more appropriately sized over-current protection. I'd use an Edison-base breaker, rather than a fuse. Only problem is ... they didn't work out.
They didn't work out, as the first thing the HVAC guy did on his next visit was remove them, leaving an open box in place.