if you use the search, there was a long thread explaining, but
IMHO means "in my humble opinion". So one might say "IMHO, Colin is totally wrong again". This "reads" much better, and since nobody can guess your mood, it clearly highlights that it is an opinion, not necessarily fact.
Likewise, sarky comments should end with a smiley thus ;-) to illustrate (this time) that the comment is meant v nicely, not nastily.
Copious use of these makes you seem a pleasant chap. Non-use means it's not easy to guess.