Even though Apple talked about general changes and big new features of MacOS Sierra during the WWDC 2016, they did not talk about every single feature. However, as MacOS is currently in a beta, users are discovering new features.
Anyway, the Menu bar is a significant part of OS X ( and now MacOS ) because it is the screen where you see the options for apps; it is the screen where you see your battery indicator, time, and settings for wifi, Bluetooth, AirPlay, etc; also, it is the screen where you see shortcuts forThird-Party apps.
Normally, you can arrange the official menu apps by simply holding the Command key and then dragging the app. However, the same thing couldn’t be done for the Third-Party apps ( at least, not officially. You could always use some apps like Bartender ) until now. With MacOS Sierra, you can arrange the stock apps as well as the Third-Party apps on the Menu Bar.