Elevated water tanks are elevated to provide water pressure. When a water tank is elevated, gravity forces the water to press downward as it drains through a small pipe. It is the weight of the water pressing down on itself due to gravity that creates the pressure. This allows the water to be forced through the water system at high speeds and provides enough momentum for the water to reach homes that are located many miles from the water tank.
Safety is another reason water is elevated in a tank. An elevated tank is more difficult to reach. This prevents someone from contaminating the water supply. It also prevents hazardous spills from seeping into the tank through the ground. Water is a resource, so suspending the water above the ground also reduces the risk of accidental puncture by a car, truck or other force, which is more likely to occur on the ground.
Another benefit is cost and energy savings. The elevated water tanks provides it's own pressure to propel the water into homes. The cost to manually pump this water into the homes from a ground storage tank or reservoir far exceeds the cost of pumping the water up into the tower. The reason is that water would have to be continuously pumped into homes, through a maze of piping systems so the pump would have to run all the time. With an elevated water storage tank, the pump only has to send the water up to the top of the tank. The tank can also be filled only when the water level runs too low to provide enough pressure for the system to work. This is true for large municipal systems, and home water tanks.