R* put a gazillion of "useless" (guess that's a matter of personal likes rather than a matter of fact) things in every GTA. Why not functional gas stations? Adds some more realism to the game, just like the car wash did and the friends-system did and the minigames did an so on...
and once you get 4 stars a tank of gas is no way near good for 300 miles, I doubt you run from the cops at 35mph