"IMO" is a "softener". While it does not add any important primary information to a statement (because most people should be able to tell fact from opinion), it does add meta-information to a statement communicating that the writer acknowledges the fact that other points of view exists and may also be valid. Or short: it makes statements less confrontational. "IMO" adds nothing to the content but to the way the content is delivered.
It's mainly an Anglo-American thing. Americans and, to some extend, Brits and Aussies are obsessed with the usage of softeners. As a German, I find it quite anoying because it often adds more or less useless fluff to the language and cultivates a certain touchiness in people. If you're not using softeners you're considered blunt or even rude. This has been an endless source of frustration in German/American cross-cultural communication because Germans do the exact opposite: we make extensive use of intensifiers ("absolutely", "without doubt", "must").
Someone who grew up in a German language country might state an opinion like this:
"We must absolutely change the way we approach the problem".
An American would probably put it this way:
"In my opinion, it may be a good idea to consider alternative appraches to the problem".
It's actually one of the sources of many of the more persistant cultural stereotypes, like "German are arrogant" or "Americans are only superficially friendly but don't really mean any of it".