In Microsoft Word for Windows it's the "Styles" bar, and on MacOS it's the "Styles Pane".
It would be interesting to see a word processor that enforced style usage...
And sure, it takes some discipline to use styles, but that goes for any other option as well (apart from things like Docbook maybe). You're supposed to use \em in LaTeX instead of \textit for the same reasons you're supposed to use an Emphasis style in Word instead of italic text. The only difference is that the mechanism is the same in LaTeX (type a few characters – which is pretty much why for any given macro you'd be hard-pressed to actually decide whether it's semantic or presentational), while in Word it's a separate feature.
I'd also be surprised if word's implementation was anywhere near as powerful as a full-on search replace on latex using regexps.