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.