Fair warning, it's a bit long:

Note - this post will contain my basic thoughts about Christian Atheism, the evolution of religion (i.e. via natural selection), and the connection between them. The connection between parts 1 and 2 may not be immediately evident, but stick with it and I promise it'll make sense (probably). TL;DR near the end.

I'll start with a definition of Christian Atheism. This is probably not totally accurate, but it'll serve for the purposes of this post. Essentially, my personal brand of CA holds that the Bible is relatively true, but God is just a metaphor for inherent goodness, and Jesus was either just another part of that metaphor, or part of that metaphor which was conflated with an unusually wise teacher. There are a couple other aspects like heaven being just the state of having lived a good life, but those are the basics.

I find such a simple definition is useful because it allows CA to co-exist with whatever one's beliefs about the rest of Christianity are. For more on the separation of Christianity's belief system from its morality, check out Peter Rollins's description of one thing he feels the church as a whole gets wrong from his AMA here. [Note - removed link because the AMA took place in a private subreddit]

OK, now that that's done, a little about the evolution of religion. I find arguments for God that claim a belief in a higher power couldn't have developed via natural selection entirely unconvincing. Morally speaking, religion is advantageous because of group selection. Basically some theories of group selection hold that while egoistic individuals outperform altruistic individuals, altruistic groups do better in the long run. As a result, groups ended up with belief systems that held altruism in high regard. For more on the debate, see the IEP's page on Altruism and Group Selection.

Further, in terms of non-moral beliefs about the afterlife, religion is advantageous because, as Ernest Becker argues in his seminal work The Denial of Death, if we hadn't developed ways of dealing with our awareness of our own mortality, we would likely all go mad. So we developed systems that allowed us to deny death, including religion. While it's debatable whether we would actually all go mad if it wasn't for death-denial, it's much less controversial to claim that simply denying our death religiously (pun absolutely intended) is much less resource-intense (mentally) than coming to a place of acceptance of our death, and this is all that's necessary for death-denial to be selected. The important things to recognize here are that a.) religion as an ethical system need not be irrational just because belief in a higher power / afterlife is and b.) these two aspects of religion are independent in terms of their evolutionary benefits, and don't necessarily need to be part of the same system (i.e. religion).

This is where CA comes in. In accepting the moral systems of Christianity but rejecting its (arguably irrational) beliefs in God and an afterlife, CA separates these two evolutionary aspects, allowing for (arguably) a more rational form of religion. Christianity's beliefs in God and an afterlife are irrational and make unfounded (and fundamentally unprovable) claims about the world, so it makes sense to reject them and find another way to deal with death-awareness. However, the moral/ethical system Christianity espouses is generally advantageous because it allows society to function more effectively (through loving/forgiving one another, giving to the less fortunate, etc), so there's no reason to reject the latter aspect just because the former is irrational.

TL;DR Christian atheism is useful because it separates Christianity's moral/ethical system from its death-denial, two distinct evolutionarily useful aspects which happen to have been conflated into one system.