A combination of your safety and the most economic solution to a problem.
The runway at Skiathos is too short for a fully laden A320 to go direct to Gatwick, so you will take off light and make what is a short hop to refuel.
Better than the practice of some UK airlines in the 1970s and 1980s which would file for a destination, in the knowledge that the range with the fuel on board was marginal to stay legal with enough fuel for a hold at filed destination and diversion.
If the winds were OK they would make their destination, if not, they would divert en route.
Airlines operating from southern Spain and the Canaries to Glasgow using the then Amber 25 were regular abusers of the system and frequent diversions to Bristol, Cardiff and Manchester resulted in the CAA banning the practice and the fuel stop had to be taken in the first half of the flight.