The little pocket on jeans is a watch pocket, originally designed for pocket watches in the 1800s. The pocket was meant to hold pocket watches, which cowboys used to wear on chains and kept them in their waistcoats. The pocket was first used with the original Levis "waist overalls" jeans in 1890. The small pocket was meant to hold pocket watches, so you wont usually find it on suit pants, which already have pocket watches, making one on the pants superfluous. The pocket is still around today, even though people dont use pocket watches anymore, as an homage to the pocket watch and to maintain the integrity of the original design. Different brands put the small pocket in different spots, with some putting it in the front, next to a bigger pocket, and others putting it in the back. Today, people use the pocket to carry small items like coins, AirPods, emergency pins, and hair bands.