WPF MVVM Why use ContentControl + DataTemplate Views rather than straight XAML Window Views?

People use DataTemplates that way when they want to dynamically switch Views depending on the ViewModel:

<Window>
    <Window.Resources>
       <DataTemplate DataType="{x:Type local:VM1}">
          <!-- View 1 Here -->
       </DataTemplate>

       <DataTemplate DataType="{x:Type local:VM2}">
          <!-- View 2 here -->
       </DataTemplate>
    </Window.Resources>

    <ContentPresenter Content="{Binding}"/>

</Window>

So,

if Window.DataContext is an instance of VM1, then View1 will be displayed,

and if

Window.DataContext is an instance of VM2, then View2 will be displayed.

Granted, it makes no sense at all if only 1 View is expected, and never changed.

Since in VaD the view models know nothing about the views, you can build a fully functioning application entirely made up of view models only and no views. This leads to the possibility of writing an application that can be driven entirely by code. This in turn leads to the possibility of performing integration testing without the GUI. Integration testing through the GUI is notoriously fragile - while testing through view models should be more robust.