Generally speaking it is for safety reasons to speed up evacuation in case if plane catches fire during refuelling.
Although such incident rarity, passenger safety is priority. If plane do catches fire, fire can spread pretty quickly, engulfing the aircraft with smoke, fire and high temperatures within seconds.
According to Airbus, each year, thousands of fuel spillage events occur worldwide although since 1961, a total of 8 aircraft were damaged during refueling. The types of damage include cases of limited wing damage to cases of aircraft destruction.
Most recent incident that involved refuelling occurred in February 2016 when aircraft fuel tanker caught fire while refuelling an Etihad A380 aircraft at Abu Dhabi International Airport (AUH). No injuries were reported.
So, next time when you are asked to fasten or unfasten your seatbelts, remember, it for your own safety and others.