I don't know that many atheists have defined or discovered their purpose with great clarity. I think some have, of course, but mainly because it's a point that's central to the god debate. I think I'm in that camp, and I've thought long and hard about what "purpose" a purpose serves, and whether or not I'm following a compass without realizing it.

I have a couple core purposes that I've discovered drive me, and I'm still exploring them. One is to raise my kids to be the happiest and most productive people they can be. The second is to promote human flourishing in a sustainable way. The second is more of an umbrella, but I see this one poking through in all aspects of my life. I also see that where my behavior deviates, that behavior always seems to belong to a person I dislike, when I analyze it introspectively. The key to the second purpose is sustainability. There's a lot of hidden danger in taking short term gain and being blind to what comes after.