Sometimes you just need to emphasize something. One of the best ways to do so when you’re texting is to make the words you really need to get across in all capitals. Or maybe you just want to shout at someone, and an ALL CAPS sentence will certainly get that across for you.
Before now, I’ve always just deleted the word I was trying to emphasize and re-typed it after double-tapping the Shift key in iOS (for Caps Lock). Now, however, it looks like you can change the case of the word after you’ve typed it without deleting anything.