Many people on the internet seem to have problems discerning opinions, so stating that it was your opinion can make a big difference. For example:
"Normchacho is cool." - This statement could be seen, according to some people, as trying to state an opinion as fact.
"In my opinion Normchacho is cool." - By adding IMO you not only make things clear but, in a way, you are implying that this statement only works from your point of view and could be wrong.
(Note that implying a statement is an opinion could also be considered "slander" if it's understood that said statement is actually factual and not mere opinion).
“Simple minds have always confused great honesty with great rudeness.” - Sherlock Holmes, Elementary (2013).
"Did you guys expected some actual rational fact-based reasoning? ...you should already know I'm all about BS and fraudulence." - FunFan, VGchartz (2016)
