This is most common on the 737-300/500 series aircraft. On these aircraft, due to cost-saving measures, water pressure isn't available on the ground due to the pilots receiving instructions to keep the APU off (this is also why it's so hot now when you get on a plane... if enough of you write the company letters, maybe this will change. We hate this just as much as you do!!!), so these are in place to allow you, the customer, to wash your hands after using the restroom on the ground.