I don't grasp the benefit of this approach. You have a gauge that (albeit not particularly accurately) tells you how much fuel you have left, and in doing so,

I don't grasp the benefit of this approach. You have a gauge that (albeit not particularly accurately) tells you how much fuel you have left, and in doing so, gives you a clue as to when to fill up. It would be like working out how much to pay for heating by remembering how warm you keep your house, and how good your insulation is. Why not just look in the little cupboard and get the number off the meter?