I don't know what the reason is on here, but it is very irritating to me when I used to see it on Face Book. Usually it means you are stresing a point, and some say it is "screaming" the answer. Personally I don't do it, and when I see an answer on face Book I skip right past it. Irritating.
Supposedly it is considered RUDE. Who first established this "rule" and why is beyond me. (It was probably the brain child of the same type of "GENIUSES" who think Daylight Savings Time is a good idea and all the minions who mindlessly go along with such absurdities. I use caps on here quite frequently when asking or answering or even commenting because I want to put special stress on a word and also because ITALICS is not available as an answering option. If I want to say something "loud" on here, I don't necessarily need to use all caps. Ones "tone" can be heard, with choice use of words that do not really need to be typed in all caps. Still, YOU can SHOUT at ME, all you want, because capital letters do not hurt my eyes or feelings.