I would think that opinions are tools for solving problems. An opinion is a hypothesis that gives you information about a situation so you can understand it, gain an advantage, and manipulate it. But then opinions start to have other significance as they become a qualification for group membership. I can't be a liberal or republican or college student if I don't subscribe to certain opinions. Every opinion is constructed to gain something.
As for why we defend our opinions, I think you're right that we want to know that we're interpreting the world correctly, but even more so, I think an opinion can serve as the justification to some resource, right, or property. In politics, for example, the Palestinians have formed the opinion that they have a right to return to their homeland in Israel. They want the world to side with them so they can establish a right of return. But the Israeli government espouses a competing opinion that there is no right of return, presumably because granting a right of return would affect demographics and voting and threaten the integrity of a Jewish state. So, the opinion serves as a bridge to the resource. Same goes for relationships. In the end, it's about access and power.