As in, how is it useful? You have no pointer, so you have no way to interact with anything on the external screen, so what's it good for, other than mirroring, which AirPlay already does?
The best they showed for an external screen was editing video. It wasn't a direct mirror. As you pointed out, however, this is also already achieved with AirPlay.
At least it introduces the possibility of connecting a display and as iOS continues to improve it can offer advanced, pro-like features, better taking advantage of the possibilities opened by USB-C. Time will tell but at this point, it's the software that lags. If that doesn't work for you then you should be looking at Mac and MacOS.