As evidenced by this question, I am new to the gas plane world and the use of pumps for fueling is stumping me. I am used to chainsaws and the like where you open a cap and pour in gas. Why is it standard for people to use fuel pumps (be they electric or hand crank) in line with a fueling line?

1) Is gravity not sufficient because the fueling lines are of such small diameter that it would simply take forever to fill?
2) If this is true, could I, a cheapskate, use a bike pump or similar to pressurize the filling jug (attach a schrader valve or something) rather than buying a specialized pump that goes inline with the fuel line?
3) Or some other option. I would even just add a screw cap to the top of the tank, but it seems like tanks don't come that way.
4) Or I should just buy a handcrank...