Here’s my take on this issue.
A lot of what I write online is personal in nature – my opinions that are borne from my own experiences. It is how I feel and think about certain issues, based on what I have been through (or not been through).
So when I am sharing opinions I am really just sharing experiences, because I know how it feels when I read about other people’s experiences – often, it makes me feel less alone. And I want to be able to do that for others with what I share.
When I try to write more persuasively, I know I cannot change the minds of folks who are firmly entrenched in their viewpoint. I think often, though, some people who are on the fence about certain issues read the exchanges between two opposing sides and then make up their minds from there.
It is that audience I write for in those cases – I figure if my argument is sound and well-reasoned, someone will read it and go “Yeah, she has a point there.” I know I have read stuff and then seen things in a new light.* I have become more educated and aware through reading online debates (even the nasty ones), so I know it can happen.
*Not that I feel it is my role to “educate” other adult humans, nor do I think my position is always correct – I am just pointing out that some reader’s minds can be changed.
And the third reason I sometimes get into online debates is because it helps me clarify my own position – on occasion I have even abandoned my own position (changed my mind) because I realized in the course of the argument that I was an ill-informed idiot on that topic after all.