The remarks of the documentation for that property explains it. You probably didn't see that. We'll try and add some more info elsewhere in the docs on this. But here's the info:
Normally, setting the Window.WindowStyle property to None would hide a title bar. However, RibbonWindow is often forced to be in None style to achieve its custom UI. Therefore, set this property to false in cases where you maximize the RibbonWindow, hide any child Ribbon and want to achieve an effect similar to normal windows when their style is None. This sort of functionality is useful when trying to mimic a "full-screen" mode temporarily.