For a very long time, China and India used a mixture of the 12-hour and 24-hour formats, which meant they had to convert the time or switch between time formats multiple times par day. That was the intended use of this button.
Both countries have now officially switched to the 24-hour format.
I'm skeptical. Switching between time formats when you're used to it requires 0 cognitive effort, and certainly doesn't warrant a button. Eg in France, people use 24h time for display but talk in either 12h or 24h time depending on personal preference, so you might look at "17:00" and say "5" without even thinking about it, it's just a different name for 17.
It’s not quite the same when a schoolchild or a farmer has to figure out different train timetable formats depending on the railway company. It was the 1980s, and these were the first affordable watches available…
The SNCF doesn’t change its format when you travel to Brittany or Alsace.
It doesn't take long to learn to interchange. If you start today you won't be doing it tommorow, but if you keep it up in a few months you will. At least for the common times - you might have to think about what 10pm is if you normally go to bed before then.
Both countries have now officially switched to the 24-hour format.