Uplinker
Some airlines mandate the presence of a fire engine during refueling with pax on board too - not only airports.
I thought the main reason regarding "the remain seated with your belt undone" instruction was to help evacuation (no seat belts) by keeping the aisle clear (remain seated). I have done it once and it was very annoying since I kept fastening my seat belt. I had a developed sense of aviation safety even some ten years before joining pprune.