Your in New Orleans after Katrina, or in Virginia after the big storm, or in CA after the big one, or...and the water stops flowing from the pipes.
Safe drinking water is a primary essential. Guess what, if you have a water heater, even with the water pressure gone, you have a tank of potable water. Every water heater has a drain on the side at the bottom, and it is threaded to fit a garden hose most of the time.
You have (however many gallons you tank size is) a ready supply of safe drinking water sitting right there in your house with you. It's not something a lot of people realize.
It won't last forever, but it will help for a few dsys.