Cycling jerseys, by tradition, have at least three rear pockets. These are useful for keeping keys, phones, or any tools such as pumps and spares you might need on a ride. It's therefore desirable for the pockets to be made from quite an elastic material to prevent the pockets from sagging.
Some jerseys also include a zippered pocket which can be very useful for keeping values, providing peace of mind that there’s no chance they’ll bounce out.