No doubt as some of you might have already experienced, some watches have this feature in their watches.
And the lingering question comes to mind: Why bother having a 24-hour complication? Mind you, this is the type that doesn't move separately from the main hour hand but moves in sync with it.
I've searched for reasons as to why Seiko would include it in their VK chronograph movements and Miyota in their cal.824x series but couldn't find a clear cut answer. Sure you could treat it as an AM/PM indicator. But unless you live somewhere where there's only 1 hour of sunlight, underground, or on a space station, it wouldn't be all that useful for the rest of us.
It does help to indicate when your date will about to switch over, other than that, there's not much to it. Perhaps it's a vestigial complication in that since it was already there, why not let watch brands have the pinion and do something creative with it?
Some watch designs even made a pseudo-regulator watch with it. But using that small sub dial with even finer hour indicators doesn't lend much to readability. It's quite novel though, so there's that.
I guess we'll never truly know why it's there unless a knowledgeable watchmaker comes by to enlighten us all.