While it is true that this is dead simple to implement (and probably exists elsewhere. I know there are plenty of side-by-side editors that require you to push a button. I bet one realized they could eliminate that.), it's still very useful for people learning HTML, wanting to try some obscure thing with HTML super-fast, etc.

Easy to implement != useless or boring. Look at Twitter.

1. Noteworthy lies at the intersection of "useful" and "unique." "Complicated" and "difficult" doesn't come into play.

2. Because it's something immediately useful to which I can direct anyone learning HTML so that they get instant feedback.

It's noteworthy because it says "instant".