A trigger is only a trigger because mechincally the digit needs leverage. The actually input requirement is digital - shoot or don't shoot.
Actually maybe not 100% true. Simulating different kinds of trigger action, determining whether or not you want the trigger to fire at 10% depression or at 90% depression. And that does help to some extent if you'd want to simulate the trigger action of a variety of guns a little more - you could put an automatic gun at 10% and an old six shooter at 90% for instance (I have no idea if there are already games that do this).
And that the digit needs mechanical leverage, well, maybe the 360's controller versions of 'R1' and 'L1' prove it by not working very well (my L button barely works at all), but the difference between those and the triggers below them couldn't be bigger!