Well, today, it doesn't really serve a purpose, but it used to in the late 1800s. The small pocket is called a watch pocket. Back then, cowboys used to wear their watches on chains and kept them in their waistcoats. To keep them from getting broken, Levis introduced this small pocket where they could keep their watch.


Reference: Google

Cc: Lalasticlala

I think it's a wonderful accessory

It is known that this pocket first appeared on Levi's 501 jeans. There are several versions of why it was invented. According to one of them, the pocket was used to keep watches on a chain in it - then, it was a bit bigger than now, so the watches could easily fit in it. By the way, in Levi's catalog, it is still called "watch pocket."
Now the retro style is in fashion, and many people are interested in how to wear a pocket watch. I also like to wear such a watch, but I like to combine it with suits and pants.