I think that’s open to interpretation and context really. I see no reason why you wouldn’t want to be able to style a semantic element like nav or aside. Could be any number of reasons, from padding to background colour (yes you could apply those to the ul in this context).

With nav element, it can contain things other than lists, like headings and text. So if we wanted to do something like background colour in that instance, you have to add an arbitrary element to target.