Personally

I think people do that to acknowledge that there may be another point of view. I personally hate the H bit of IMHO. My O is not H and never will be - always seems a bit too deferential IMO.

Sorry - yes, I do take your point, but I do think it's a way of allowing the other person their view. It reminds me of "With all due respect ..." then rubbishing whatever you are disagreeing with.

Language is a strange thing, for sure.

Don't even get me started on "So ..."

I agree it is overused, but we teach pupils to use it stylistically a lot in essays to introduce their point of view. "Personally, for me, according to me, in my opinion," followed by "I think" or "I believe" are very current expressions which add more weight to the argument and certainly gain more marks in essays and in speech. Pupils have to learn these expressions in all languages.
Much of the time I myself personally think people just use the expression to pad out what they wish to say.