Mrebo said:

I've made the point before, but it's simply weak phrasing to say "in my opinion" or "I think," when expressing an opinion. It's useful if one really needs to make clear something is opinion or to express doubt about one's position on something (I think his car is blue), but in general it's an unnecessary disclaimer, I think. See how silly that is!

I think it's helpful in clarifying that the person giving the opinion recognizes that it's an opinion, and not an objective fact. After all, it's no fun giving a POV on something only for another person reading it to jump to the conclusion that you're a conceited blowhard.