What is the bid deal about typing in caps?

Its a Big deal cause back when programs like MIRC(still used by some) were used one would chat in lower case and if they got mad or angry at another in the chat group you would use caps to show the anger. those of us that have been around recognize this still though. A lot of us just know most don't know the old stuff and we ignore it. But there are a few out and about that still get mad at caps. Guess they don't know how to change.