system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the purpose of water towers and why do they need to be tall?

I just drove by one and for the first time wondered what they are really used for.
gpt
With a water tower you need a smaller pump and it helps regulating the pressure. Basically they have the water come in, and they pump it at pressure into the system. Actual flow is dependent on use, so in the morning everyone takes a shower and flow is high, at 2am everyone is asleep and flow is low, throughout the day it varies constantly. What a water tower does it fills with water, storing it and keeping the pressure up. During high demand periods the water tower can provide the high flow high pressure water by simply letting it flow down. Then your pumps only need to keep the water tower filled and they don't actually need to be able to meet peak demand and they don't actually need to be on all the time, once the water tower is full they can turn off and let it drain. That also means a pump doesn't have to deal with variable flow rates and doesn't actually need a pressure regulator. Additionally if the power goes out you don't automatically lose water pressure throughout the town since the water tower can provide it.

Without a water tower you'd need all pumps on all the time, and their flow needs to very throughout the full range of demand or you risk losing water pressure in the system (which is a health risk, leaks can flow backwards and let nasty stuff into the pipes), a power outage or maintenance on the pump could force you to shut everything down and flush the pipes.

Also, you don't need a water tower specifically, people with well water will usually have a tank filled with a bag of pressurized air so it's regulated by air pressure instead of height. This allows your well pump to turn on and off, again, because you don't want the pump always on and those things don't have tight pressure regulation.