/r/atheism has a lot of purposes, most of which fall under the catch-all purpose of "we want to talk about atheist stuff with other atheists".

I live in the south and work in a place where, if I were to reveal I was an atheist to my co-workers, I would be unwanted by at least 50% of them. These are people that listen to praise/worship music on their speakers, post pictures of children they sponsor on the walls of their cubicle, walk around the office quoting scripture. These are people that would absolutely think less of me if they knew I was an atheist. Some have taken sick days to avoid a simple hour long halloween party. I work in a service position for these people and wouldn't be able to do my job if they acted differently toward me as an atheist. Granted, I don't think religion has any place in the office but that's clearly not how they feel. There aren't too many places where I can go and have people empathize with that position.

I grew up christian and was an evangelical christian until I was about 19. I bugged my friends about their salvation, went to christian youth leadership training, christian youth festivals. I went through high school believing that any lustful thoughts were evil and I needed to basically just avoid women altogether. As an atheist now, I feel like the adults in my life at the time shaped my childhood in a way that was at best disruptive and at worst destructive. There aren't a ton of other people that can empathize or even understand that outside of other atheists who experienced a similar childhood.

At one point I was a very weak Christian who had basically completely disconnected with the church. I tried to shoehorn God into every gap available. I tried to reconcile the creation myth with evolution. I tried to reconcile the massive amount of contradictions in the bible. I tried to reconcile my feelings with the Church's various social stances. I subbed to /r/atheism because I wanted to read more about it. /r/atheism showed me, and many others in the same position, that I could be "Good without God"

Those are just a few of the purposes of /r/atheism. Yes, in the end /r/atheism is a MASSIVE circlejerk but it's a circlejerk with a purpose. As a subreddit we raised $180,000 for Doctors Without Borders. A lot of that was thanks to a massive circlejerk of "I'll donate $0.05 for every upvote" self-posts a few days ago. Again though, a circlejerk with a purpose.