Whereas I don’t understand why anyone would need for someone to state their opinion is in fact an opinion.
You don't? Hopefully I can enlighten you.
Speaking broadly about human psychology (so, there are exceptions) - the words we use matter. How we speak about things changes how we think about them, the emotions we attach to them, and how others think about them.
The most basic example which is perhaps most relevant to these messageboards - for human beings, the more authoritatively we state a position, the more compelled we will feel to defend that position, up to and including making crap up to not have to back down. Backing down is seen as a threat to one's position in the social order, and to our hindbrains, that's not really different from a physical threat.
The more we explicitly build pressure releases into our statements - "In my opinion" and "as far as I understand" and so on, the less we feel a need to defend the position. And, the less others will feel a need to attack them. And, when someone does question a position, we will tend to give the new information more honest consideration if we have already openly admitted our position isn't necessarily absolute truth.