One important additional function of the cities chapter ring which you didn't mention is checking the time of any other world time zone. To do this, align the chapter ring city with the time zone which the GMT hand has been set to. You will now be able to read the time in any time zone in the world from the 24 hour scale. Since the chapter ring doesn't move automatically, you need to align it manually whenever you want to check it.
One important additional function of the cities chapter ring which you didn't mention is checking the time of any other world time zone. To do this, align the chapter ring city with the time zone which the GMT hand has been set to. You will now be able to read the time in any time zone in the world from the 24 hour scale. Since the chapter ring doesn't move automatically, you need to align it manually whenever you want to check it.