You asked:
How does a water tower work? — John, Fishkill, N.Y.

Marshall Answered:
Think about the way human beings operate and you will understand that there are different demands on the water system at different times. For example, at 3AM people are not using much water in their homes. But at 7AM there is huge demand because everyone is taking a shower. The same kind of demand spike happens after everyone gets home after work.

A water tower is basically a huge tank stuck up in the air. A pipe connects the tank to the water main. At 3AM, when there is low demand, the water tank fills up. At 7AM, when the showers start, that stored water flows into the system and meets the demand. It lets the water company buy smaller pumps, and also makes repairs/maintenance on the pumps possible without stopping the flow of water.

For more info see: How Water Towers Work