Why do opinions exist? Because there is discord, wonderful delicious discord. (of course, there is discord that is not beautiful as well, but let's focus on that elsewhere). Why do people share opinions? Some to better form them, some to test them, some to change other people.... or all of the above. None of the above? What is left?
Opinions also create self through meaning. Try that on for size. :) Or don't. I think you should. But perhaps I have an overinflated sense of self.