I'll skip the rant, and answer the Islam.SE-related questions:
Why does this community even exist ?
Take a look around; we're here to learn about Islam. As long as you play nice with others, it's fine to be an atheist here; you don't even have to like Islam to participate.
I personally don't like homeopathy, yet the journal Homeopathy gets published by Elsevier, who also publish journals containing some of my best papers. I guess I don't like something is not reason enough for it not to exist.
For preachment purposes ? That's all I can figure out.
Preaching is not the intention of the site. (See e.g.: Islam—Stack Exchange is not for debates or apologetics.) That being said, (a) studying Islam will likely cause people to develop a more (dis)favorable opinion, and (b) preaching seems to slip in from time to time.