The ground wire between the hot and cold water pipes is called bonding. The NEC 250.104 requires that metal water piping systems "shall be bonded as required..." The same principle applies at your water meter as there is a jumper around the meter in case the meter is removed to insure that the electrical system stays grounded. In this case when the hot water heater was removed the hot water piping system would be temporarily ungrounded if there was no bonding wire between the cold and hot water piping systems. Sometimes hot water heaters used dielectric unions because of dissimilar metals so you cannot assume that the hot water piping system is grounded through the hot water heater itself.