Surely you have wondered why jeans have a small pocket inside the front pocket. Some will say they are pockets to hold loose change and there are people (men generally) who use them this way. Others think they are to put the lighter or tickets or other useful and small objects that they carry around.
But actually the pockets were for a very important purpose when the first jeans were created. Think that jeans date back to the 1800s when all men had a pocket watch. It was therefore essential for this new type of trousers to have a special place for this object that all men then had. Now practically no one has a pocket watch, but the pockets are still there.