Traditional Door Design Query

My question is why the traditional panel door design features tapered tenon shoulders on the mortise and tenon joins between the middle rail and the stiles. Now I think it looks great and definitely adds elegance - a bit like tapered columns - but does it serve some more essential function? It would seem to me to be much more difficult to make that way.

Edit I've since learned the style is called gunstock doors or gunstock stiles. Is there a structural function the gunstock stiles serve?

Cheers, Zac

I believe it simply a styling design and provides more glass area than other panel door designs.