[frosted-flakes]

"I think" is not always redundant. It serves the same purpose as "in my opinion" or "I'm pretty sure", which is to temper the confidence of the statement or indicate an unverified intuition.

Example:

"Hey, where's the emergency toilet paper supply?" "I think it's in the cupboard under the stairs."

I think (but am not sure!) that this use of "I think" is intuitively obvious to most native English speakers, especially when paired with voice tone.

Obviously, if you're sure that a statement is true, then adding "I think" only weakens your point. Unskilled writers might not know to avoid it in (say) an essay, which may be why you heard that advice in school.