1. I rarely fully understand my own positions on minutia 2. Writing is rewriting.
I write forum posts to solidify my understanding of my own interests, beliefs, and reasoning. I often edit them multiple times before moving on and ignoring the responses thereafter. I can reference them and have to other people who ask my opinion. Sometimes I do respond back to replies immediately, and sometimes I revisit days later, after I've had time to put it in my day-to-day context. It's not a hard and fast rule.
Posting stopped being about convincing someone else maybe 20 years ago (around age 30). I do post to look back and understand myself. To others, I'm sure this sounds like existential navel-gazing and self-centered blathering, but I don't mind.