Curious people over at the Quora discussion forum have found out that the tiny little pocket inside a pocket was for watches, designed for cowboys in the 1800s.
On the Levi Strauss website they say:
The Watch PocketSince we are in 2016 and we are, for the most part, not cowboys, these pockets have taken on new guises. I've been using it myself to hide small valuable stuff such as a a neck or wrist chain, coin, neatly folded paper money, a key and I don't remember what else. Click the link above to visit the site and find out many other use of this pocket.
The first blue jeans had four pockets—only one in back and, in the front, two plus the small, watch pocket. Originally included as protection for pocket watches, thus the name, this extra pouch has served many functions, evident in its many titles: frontier pocket, condom pocket, coin pocket, match pocket and ticket pocket, to name a few.