What is it?
I currently view it as 1) an option for theists to consider, and 2) as a way to help mobilize separation of church and state movements.
When I first identified myself as an atheist, it was due to the "last straw" of GWB's pandering to the religious right, which I'm quite certain is what also enabled our knee-jerk invasion of Iraq; the righteous Cheney, et al kabal. Now look at the power of religion, as in ISIS! And the forced closings in some states of abortion providers. (On the positive side, there are the emboldened politicians like Cruz and Huckabee nudging Republican primaries into unwinnable general election outcomes. Yay, partial payback for Naderism screwing up Gore's effort to pilot our course in world events!)
Really, what is the purpose of Atheism? Let's realize there is no official purpose, so it's really up to each of us individuals out there claiming the name.