I think the primary goal is the exchange of ideas. I like people to know what I think, which is not always practical in the real world for a few reasons - - people don't like to have serious political or religious arguments when they're really just looking forward to a light, friendly get-together, people get angry and hostile when presented with new or different ideas, which makes it unpleasant for the other company, etc. I feel that in the anonymity of the internet, it's good to meet people from other backgrounds, who I'd probably not have the chance to meet in real life, blow their minds with some of my own thoughts, see how they think and especially how they react to thoughts they aren't accustomed to hearing, and see to what extent, if at all, they are surprised or taken aback to rejoinders or rebuttals they never anticipated. The cut and thrust of debate, I guess you could call it.
A secondary reason, and I'm not sure that it's really legitimate, so I'm trying to cut it down if I can, is to vent. Sometimes I'll see something that just makes me so angry, sick or disgusted that I have to vent those negative emotions in a forum which I perceive to be largely supportive of whatever it is that is agitating me at the time. I guess it's a symbolic venting at best - - ideally, I'd like to tell the person directly involved as perpetrator just how outraged I am at his (it's almost always a "him") conduct, but lacking that opportunity, I have felt that the next best thing would be to say so to persons who I perceive to be supporters of that conduct. This seems to lead to emotional exchanges, but in a way I think it's taking advantage of the other members, and, as the saying goes, generates more heat than light, so I think that while such exchanges can generate some mutual understanding, it's better to keep a neutral tone when entering into them.
And sometimes I just think it's a good place to crack a joke if I see an opening for one in somebody's post.
I used to think this would be a good place to convince conservatives of the errors of their ways, but that was a long time ago - - I don't think I've ever convinced anyone of anything in this forum, and apart from partial-birth abortion, I don't think anything here has changed my opinion on any issues. Still, there are some statements that I regard as so awful that I feel morally bound to add in my two cents' worth just so they do not stand uncontradicted on the record, even when I know that no minds will be changed by what I write - - I regard that as a kind of moral obligation.