This thing is that you cannot highlight a portion of text then press a button and either capitalize all of it or uncapitalize all of it. This would be extremely helpful if you had capslock on by accident and didn't notice your mistake until after you had typed a lot already. In this scenario, you'd have to erase everything you wrote, triple check that capslock is indeed turned off them rewrite it all. Now wouldn't it just be super convenient if you could transform it into something else just like you can bold a passage or italicize a phrase.
Seriously, it has always killed me that this is a thing even though it can totally be fixed.