Water towers use the gravitational head to provide constant pressure. A pump is used to pump the water up to the water tower. If a pump was the only means to provide pressure, the pressure at your faucet would be highly variable. The outlet flow rate of a pump is limited, so it would not be able to keep up with short term fluctuations in water consumption. You could build a pump system that would instantaneously keep up with varying demand, but it would be big and expensive. Instead, a small pump is used to deliver the water to the top of the water tower at a more or less constant rate. When the water is up in the tank, the pressure at the bottom of the water tower is more or less constant, equal to density*gravity*height.