There is a secondary, more nefarious purpose, near as I can tell. I constantly order items from Amazon that say "arriving tomorrow/Tuesday/etc." then the item invariably doesn't actually arrive when it was supposed to, often outside of Prime 2 day shipping even if a Prime item. But when you go to view the order online it has helpfully updated the arrival date to the new day. They push it as many times as they want to, then act dumb if you call them on it.

I've taken to screenshotting the initial shipping date, but it doesn't make much difference. I don't know what "Prime" even means anymore.

And I live in SF so it's not like there is some logistics problem with my location, either.