Occurred to me yesterday - if one wants to yell something like "ARGHHH!," one types it in all capitals. You can't really do that with the word - I - though.
I really mean it!
I REALLY mean it!
I really MEAN it!
I really mean IT!
See, you couldn't tell I was emphasizing the first word in the first sentence, but I was (<-- that one is capitalized too).