The greatest stories have something to say, about society or about ourselves as individuals. Post Apocalyptic fiction is no different. Stories about the post apocalypse, the good ones at least, aren't about the radiation or the zombies, they're about the people that remain. Those who survive the end of an old and well established world find themselves in a world that, while broken, is new and full of potential, both good and bad.
No longer bound by the constraints of the old world, those who remain are free to redefine themselves as they are able. However, just as they are no longer constrained by the old world, neither can they benefit from the safety or any commissions it once provided. You're free to be whoever you want to be in the post apocalypse, but so is everyone else.
This is what interests me about the Post Apocalyptic genre; who do we become? When the mushroom clouds are rising up to greet the comet that's hurtling towards earth and the zombie hordes are shambling towards us, who do we become and how do we define ourselves? Do we hold onto outdated grudges and ways of thinking, do we revert to some base level of barbarity in the name of survival or do we try to hold onto our core values, in spite of it all?
It links back to the old adage 'a person's true character is revealed in times of crisis.' Since there's no bigger crisis than the whole world ending, it's a pretty useful tool for the exploration of an individual's character.