It is common, if not locally mandatory that fueling hoses have an internal conductor, to bond the nozzle to the pump. And, in is intended that the fuel nozzle be in contact with the aircraft filler neck during the entire fueling. Therefore, the most critical time to bond the aircraft to the fuel supply (truck or ground mounted pump) is before the nozzle is brought to contact the aircraft filler neck. Obviously, the action of making that contact at the aircraft has to potential (see what I did there!?!) to create a brief spark. That is best away from the fuel filler neck itself. So, before getting the nozzle out, bond the aircraft, and in doing so, bring the aircraft and fuel supply to the same electrical potential. Thereafter, when the nozzle contacts the filler neck, and while it remains in contact during fueling, the potential will remain the same, and the grounding wire is redundant.
If there is a difference in potential sufficient to create a spark, it's a pretty high voltage, so pretty good at finding it's way through small amounts of corrosion, but, yes, you could have so much corrosion so as to have an incomplete circuit. I always give the ground to aircraft clip a little scratchy wiggle after attaching it for this reason.
Irrelevant to aircraft electrical systems (and they should be turned off anyway, other than helicopter hot fueling).
I'm not aware of an aircraft fire resulting from eclectically discharge during fueling. But I know that one of my clients bonds all airplanes in the hangar to hangar ground while they remain in the hangar. He'd had an airplane catch fire in the hangar (before he was my client) and they pushed it out while on fire, to save everything else. I think it was memorable for him, so now, prevention! There is a very scary security video "out there" of a fuel tanker being filled at the depot, and the contact of the filler nozzle and the filler port (being used on top of the tank) contacting. You don't see the spark, but you sure see the resulting fire!