What do any of you get out of discussing this with believers / unbelievers? Entertainment? Conversion? Usually I forum with objectives in mind. To learn of others faiths while bolstering my own faith, and perhaps converting if it is possible for some here.
Entertainment is part of it, a good civil debate can be quite enjoyable.
However, It makes me more knowledgable on the topic, exposes me to more arguments for and against Christianity and Theism, and enhances my critical thinking skills. So, it also is a benefit to me from an intellectual standpoint.
I don't really expect to come on here and convert anyone. However if that is a side-effect of my debating on here, that would be great as well.
I think religion in general is one of the most misguided, destructive forces in civilization as we know it. Doing my part to cause more skepticism towards it, if not turn people away from it would be quite fulfilling.
However, I also am aware many religious people are not willing to listen to anything that disagrees with their preconceived notion of God and the World. Coming on here with the intention to convert people would likely only cause frustration for me.
So, to answer your question... I come on here for enjoyment, and to better my knowledge and debating skills. If I convert a few people along with that, so be it.
EDIT: Eudaimonist made a great point about changing the stigma against Atheists as well. That would also be a reason why I'm here. There's an awful lot of misunderstanding, bigotry and hate directed at Atheists by Christians. Engaging in peaceful debate and discussion will give Christians a better understanding of our positions and perhaps change their minds about us as people.