Mrebo said:

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!

It isn't as silly as you think. Blue may be perceived differently between two human brains. What if my brain perceives the signal from the eye observing the same object (same wave length) as blue and while yours as what would be my brain perception of green. After all someone thought us by pointing on an object and saying "this is blue". So even if we perceive it differently in our brain, we have the same reference.

So saying "I think" and "In my opinion" is valid. :)