It's ultimately the freedom to live beyond good and evil. Obviously, religious people are generally happier (and better adapted) than non-religious people. Atheism is not selected for in a Darwinian sense, because (for the nihlistic Atheist) it has such negative effects on mental health and fertility. From my own personal experience, I recognize the loss of faith as an individual tragedy, but also an opportunity to rebuild the collective.
Gadflies like Richard Dawkins, Socrates, etc. are truly despicable because they offer no better alternative. Societally, Atheism is unsustainable. It's a symptom of collapse.