NS (darwin): What is the utility of update_window?

[Keith David Bershatsky]

It is my understanding that 7946445962372c4255180af45cb7c857f1b0b5fa provides 
compatibility with MacOS (Mojave) by restricting Emacs in such a way that 
nothing is written to the glass until the OS normally draws rectangles during 
the redisplay cycle -- forcing everything to happen with drawRect 
(expose_frame).

Other than marking dirty rectangles during update_window, does anything else 
useful occur during update_window on the NS port of Emacs?  If so, what else is 
accomplished during update_window?

BACKGROUND:  Based upon my observations, it would appear that Emacs does extra 
work on the NS port that is not needed on the W32 and X11 ports.  Whereas the 
W32 and X11 ports can update the glass during update_window, the NS port has to 
wait until a later point in time to accomplish almost same thing (but not as 
well).