Could you tell me why the author need to wrap FrameLayout for Fragment?

Code A is from activity_main.xml. and it's a part of sample project.

I don't know why the author need to wrap FrameLayout for Fragment, after I remove the FrameLayout of Code A and get Code B, and Code B works well.

Could you tell me why the author need to wrap FrameLayout for Fragment?

Code A

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:keepScreenOn="true"
    tools:context=".MainActivity">

    <fragment
        android:id="@+id/fragment_container"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:navGraph="@navigation/nav_graph"
        app:defaultNavHost="true"/>

</FrameLayout>

Code B

<?xml version="1.0" encoding="utf-8"?>
<fragment
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/fragment_container"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:navGraph="@navigation/nav_graph"
    app:defaultNavHost="true"/>

from Could you tell me why the author need to wrap FrameLayout for Fragment?