How is the StatusBarItem class supposed to be used? Is every element in a StatusBar's content supposed to be wrapped with it?

I don't really understand how StatusBarItem affects the StatusBar layout. It seems like I can use HorizontalAlignment on a StatusBarItem, but not when I put the element in the StatusBar directly:

<StatusBar>
    <TextBlock HorizontalAlignment="Right" Text="Not right" />
    <StatusBarItem HorizontalAlignment="Center">
        <TextBlock Text="Center" />
    </StatusBarItem>
</StatusBar>

Also if you wrap a Separator in a StatusBarItem the Separator changes to horizontal. Separators default to vertical when put in the StatusBar directly without a StatusBarItem wrapper.

wpf statusbar statusbaritem
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