Sunnybunny
Isn't it like putting fuel in a car ?
Eh, no it's not. I'm sure many pax believe that to be so; but there are structural limits to the weight a particular aircraft can be to taxi, fly, land and even to be loaded with, without the fuel on-board.
If you were to always fill the tanks, you would have to restrict the payload - Passengers and cargo on some flights, because you would possibly go over the max take off weight. similarly, on short haul flights you may be able to take off OK and still take all the pax and cargo, but on arrival at your destination you may then be too heavy to land! So then you would have to dump or burn off that expensive fuel before you get down to max landing weight.
Also, the more fuel you carry, the more fuel you will use to carry the extra unneeded weight.
So no, you only load the fuel you need, plus contingency amounts to cover weather and,or congestion delays and enough to divert to the planned alternate airports should there be a problem at the planned destination.
Therefore an experienced Captain, will decide how much he might want to adjust a computed figure to allow a safe and fuel efficient flight.