Why do you save the world? So ordinary people can live their ordinary lives. That was an argument made in Torchwood and, like all deeply true things, it is opposite to the way the world thinks. It feels like people out there saving the world are doing the big job, the important one, the only one that matters, but if you take away the ordinary people, then there's nothing to save. Of course, if you take away the world-savers too, then there'd be nobody doing the saving, so they're both necessary, but the reason you save the world is so regular people can go about their business untroubled.
Mokalus of Borg
PS - It also means I'm not unimportant just because I'm not saving the world.
PPS - If you follow my multi-negative point.