When to use new Screens in Flutter instead of TabBarView

I am fairly new to Flutter and I try to understand when and why it would be necessary to navigate to a new screen. Most apps keep the same AppBar, Drawer & BottomNavigationBar (if any) through all the different "screens". Wouldn't it be easier to just have one single TabBarView, or only replace the Scaffold's body ?

I have a hard time to really understand the concept of why there needs to be a new Scaffold when routing. I couldn't find anything helpful in the official Flutter doc, even the Cookbook show you a Navigation example with 2 completely new screens just to show a different Text widget inside the Scaffold's body.

Also, what about the efficiency of always rebuilding the whole Scaffold ?