title: why we have multiple stackNavigators?
body: <p>for example this code form <code>react-navigation</code> <a href="https://reactnavigation.org/docs/en/auth-flow.html#set-up-our-navigators" rel="nofollow noreferrer">documentation</a> for authentication:</p>
  
  <pre><code>import { createSwitchNavigator, createStackNavigator, createAppContainer } from 'react-navigation';
  
  // Implementation of HomeScreen, OtherScreen, SignInScreen, AuthLoadingScreen
  // goes here.
  
  const AppStack = createStackNavigator({ Home: HomeScreen, Other: OtherScreen });
  const AuthStack = createStackNavigator({ SignIn: SignInScreen });
  
  export default createAppContainer(createSwitchNavigator(
    {
      AuthLoading: AuthLoadingScreen,
      App: AppStack,
      Auth: AuthStack,
    },
    {
      initialRouteName: 'AuthLoading',
    }
  ));
  </code></pre>
  
  <p>Why we have two different stack navigators? Why we just don't wrap all of our screens into one navigator? What should be the logic for wrapping different screens in same navigators?</p>