You can live for a while without food, but you can only live 3 - 10 days without water. We have gotten so used to turning on a faucet and expecting water to come out, but what about the day that it doesn't?
There are many reasons the water supply could become toxic or stop flowing all together. Chemical spills, earthquakes, hurricanes, and the list goes on. If there is an EMP and the grid goes out, electrical water pumps will stop working. That's true for a well on a little farm up to giant water pumps in big cities.