It's to ensure there is enough to test post race and keep some as a backup sample etc.Not followed F1 for a few years now but why should anyone care how much, or little fuel there is left in the car? Seem to remember one or two drivers running out on the last lap or on the "insurance lap" after the finish in years gone by. Set a min weight, to be checked after the race by all means but the fuel one seems pointless.
I guess some teams in the past cheated and ran the car dry so the sample could not be tested.