Fuse at Shutoff

I have not seen this before. Does anyone know why there would be a fuse at the shut off? There is a 20 amp breaker for the furnace at the main panel.

The tech can service the furnace without going looking for the breaker/fuse box.

Ours are overcurrent protected by 15 amp not 20 amp, typically.

This is what we see:


Makes sense, except why have a switch and a fuse? Wouldn’t a switch suffice? Also, I could not operate the unit, the house had been winterized and the gas is off.