If coupled to a date, I mean. It helps you to correctly set the date. I mean, to know if you are in the AM or PM zone. Let' say it is 8 PM. You want to set the date. But you don't remember if you set it correctly when you set the date.
If you set the hour hand at 8, but AM, the date won't change at 12, but at midnight, as it should. With the day night indicator you avoid this kind of confusion.
I don't know if I was clear enough...

Yes, I would like to see a simple 8 Days movement, too!
Best,
Nicolas