After a disaster, if you hear reports of broken water or sewage lines, or if officials advise you of a water problem, you may want to shut off your water to prevent tainted water from getting into your home. Turning off your house valve also prevents a broken water line from draining your toilet tanks and hot water heater.
Note that the house valve is not the valve in the cement box by the street—that valve can only be turned off by the water company. The house valve is generally located outside your home near a hose bib.