The denim fabric was created in France, in Nîmes, in 1800. And it was in 1873, in the USA, in San Francisco, that Davis & Levi Strauss made the first 5-pocket jeans that you know today.
We pay little attention to the 5th and smallest of them but formerly, its function was well defined. Measuring 5cm wide, it's sewn into the right front pocket. It was originally designed to accommodate the pocket watch; round and flat, it slipped in perfectly.
It was a very useful and almost essential accessory of the time.
This is why it was also called the pocket watch pocket, it was its primary function.