As others have noted, there is "hidden water storage" in most houses that have hot water heaters, commodes with storage tanks in back or on top of them, plus a certain amount that will reside in the house piping itself. In the event that you have warning of an incoming issue/disaster as we do in Florida with hurricanes, it is wise to fill tubs, and other containers with water well before the storm is close. Having water purificaton available is always a good thing especially for those events where you simply do not have water storage available and need to "do it on the fly".