Why are DataContext and ItemsSource not redundant?

DataContext is just a handy way to pick up a context for bindings for the cases where an explicit source isn’t specified. It is inherited, which makes it possible to do this:

<StackPanel DataContext="{StaticResource Data}">
    <ListBox ItemsSource="{Binding Customers}"/>
    <ListBox ItemsSource="{Binding Orders}"/>
</StackPanel>

Here, Customers and Orders are collections on the resource called “Data”. In your case, you could have just done this:

<ListBox ItemsSource="{Binding Source={StaticResource customers}}"/>

since no other control needed the context set.