Cycling jersey-What do you need to know?

Why the cycling jersey need the pocket on the back, if you know it? 

Standard cycling jerseys have open-topped three pockets at the back for your wallet, keys, snacks and so on. You might think things could fall out of them, but in practice they're deep enough this isn't a problem, and there's usually a band of elasticated fabric across the top which also helps.

Many manufacturers have introduced variants on the traditional trio of pockets. It's common to find a small zipped pocket for keys and change, or a pocket with a waterproof lining for your phone. The three may also be different widths, with a narrower pocket to stash a minipump.