system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the purpose of water towers and why are they built so high up?
gpt
They store and provide pressure for water.

You know how you can be on the second floor of your house, turn on the faucet, and the water comes pouring out?  You don't have a pump in your basement forcing the water up, instead, you have a water tower forcing the water down via gravity.  Since your second floor is at a lower elevation than the water tower, you have positive water pressure.  If you're higher than the tower, you'd need mechanical assistance to bring the water to your floor.

*edit: 
The water is pumped up to the tower, but by nature of its size, and storage capacity, the pumps can be run when electricity demand is low.