In classic five-pocket blue jeans, the tiny watch pocket was originally designed to hold a pocket watch.
The small compartment sits inside the front pocket on many traditional five-pocket jeans. Its design reflects the period when pocket watches were common personal accessories and wristwatches had not yet become the dominant form of portable timekeeping.
As wristwatches replaced pocket watches, the compartment remained as a recognizable jeans feature. Modern wearers use it for coins, guitar picks, earbuds, keys, or other small objects, but those uses came after its original purpose.
The pocket is sometimes called the coin pocket, especially in contemporary clothing descriptions. That name describes a modern use rather than the historical reason for its inclusion. It is also distinct from the two large front pockets, the two rear pockets, and the small fifth compartment that gives the classic design its familiar five-pocket description.