"In my opinion..."
Well of course, dumbass, it's your opinion! Why would you be SAYING it if it wasn't?
Well, the person could be establishing that what they are expressing is their own opinion, and not a citation of someone else's opinion or some other precedent or established fact.
Anything you say is, by default, your opinion. It's only when you say that somebody else said it that it becomes someone else's. If I just say a thing, such as "The Baltimore Orioles suck", then obviously that's my opinion, BECAUSE I SAID IT. If, however, I want to relate someone else's opinion, then I'll credit them first. If I credit no one, then it's assumed that it's mine.