What is the purpose of a post apocalyptic genre book?
On the surface level, the purpose is to show what life is like after an 'end times' event or apocalypse occurs. This demonstrates a society that has been obliterated, and in post-apocalyptic literature, we often see a few survivors who have to learn to live under a new set of circumstances. Often, there is a reason the few or even one survives and the conditions of life on planet earth are bleak and pretty much destroyed.
In digging a little below the surface, why would an author anticipate a life beyond and after the apocalypse? There may be a few reasons. First, an author may be trying to prove that God is not the only answer to life beyond tragedy. Christianity in particular professes only one route to God or eternal happiness, through acceptance of forgiveness offered through Jesus Christ's death. It's acutally a simple concept, but human nature prevents many from believing that someone else can or even should atone for their sin. Another reason an author may propose that life beyond a worldwide tragic even is possible is to provide hope. This is a hope that life on this earth will never intentionally come to an eternal close. Yet another reason may be to promote the idea of a Christian God because the aftermath of a worldwide tragedy would be so devastating.
You have much to consider. Check out enotes.com's information on apocalyptic literature.