We recently read an article from the medium.com that explained the reason behind the tiny pocket that you often see on a pair of jeans. The article explains that that little pocket had a clear purpose - it's just that we don't use it for that reason today.
According to the article, "That small pocket originated back in the late 1800swith the first pair of jeans sold by Levi Strauss & Co, which they called “waist overalls.” The jeans debuted in 1879 with four pockets. Three pockets were on the front, one on the left and right, and the small pocket above the larger right pocket. The fourth pocket was on the back on the right. All the pockets were riveted since Levi Strauss & Co. and Jacob Davis had gotten a patent in 1873 for Improvement in Fastening Pocket-Openings. The rivets were there to make the pockets stronger. But why was there such a small pocket? The reason for the little pocket was practical, at least during the time period. It was there to store and protect a pocket watch, especially for someone working. Suit pants didn't have a small pocket because a suit jacket already had a pocket for a pocket watch."
The article goes on to point out, "The small pocket is not exclusive to Levi’s jeans and can be seen in all kinds of configurations on many brands of jeans today. The little pocket even appears on shorts. It may not be used for pocket watches anymore, but maybe someone has found a good alternative for it."
As a matter of fact, we have the perfect 21st Century use for this tiny pocket. Our ChargeCard is so small it fits in the pocket watch pocket.