Levi Strauss will tell you since they're the ones who designed jeans over a century ago and put the pocket there.
The pocket is intended to carry pocket-watches. In the past, men used to carry pocket-watches to keep time; they needed a pocket to slip the watch in so one was added to jeans.
Obviously, few people carry pocket-watches any more so the pouch isn't necessary. Instead of watches, people now put other things in their pocket, like coins, matches, even condoms if you're a high school boy. What do you put in it?
Personal fashion history -- Back in the Seventies, I thought pocket-watches were cool and owned a couple. I found slipping them into my little jeans-pocket handy. Pocket-watches aren't as easy to use as wrist-watches but they leap over ordinary watches in terms of coolness. Pulling one out to check the time always caught people's attention. Often they'd ask to look at the watch which started fun conversation.
Don't you feel smarter now?