You noticed the tiny pocket
What Is the Tiny Pocket on Jeans For?
It looks too small to be useful for modern stuff. That is because it was made for pocket watches, back when jeans were workwear.
Quick answer
The tiny pocket on jeans was originally made for a pocket watch. It kept the watch snug and protected when jeans were workwear. Pocket watches mostly disappeared, but the little pocket stayed.
Today people often call it a coin pocket, but historically, “watch pocket” is the better name.

Original use
Pocket watch
Best name
Watch pocket
Modern use
Coins, cash, small items
Still there?
Mostly tradition
Next tiny mystery
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Another clothing detail with a practical use.