That is the electrician's courtesy outlet. It is so the electrician can plug in extension cords for lights, saw, drill charger etc.
Why? Unlike some novices, pro electricians are well aware that existing houses are chock full of surprises in the electrical wiring. Bootlegged grounds, borrowed neutrals, crossed hots... those and others can result in power from other circuits crossing over onto the circuit you had thought you turned off.
The safest course is to turn the main breaker off. But then, you have no power.
So the workaround is to have one dog-simple circuit. One so simple that the electrician can effortlessly inspect it and confirm yes, this receptacle is the only thing on it. Normally they are in conduit for that reason. Then, the electrician can turn off every circuit but that one, and work reasonably safely.
Is it a Code requirement? No. The only Code that talks about this a) exempts houses, and b) is silent on the above points. It merely requires power in the vicinity, akin to outlets near HVAC units. Anything on a house can be done with hand tools. In commercial, not so much.
And that Code mandated outlet has no GFCI requirement. Yours probably is GFCI because it's in a basement, garage or outdoors - all locations requiring GFCI.