Gasoline, especially winter gasoline with plenty of butane, has a high vapor pressure and will evaporate quickly if exposed to the open air. A properly secured gas cap will keep the evaporated fuel under pressure in your gas tank and stop it from escaping into the atmosphere. This saves you money by reducing lost fuel and also helps air quality. Gasoline vapor is a heavy contributor to smog. A loose gas cap can easily emit more hydrocarbons due to evaporation from a parked vehicle than would be emitted by the running engine. This is because of the almost complete combustion of fuel.
So if you have a damaged gas cap or if you have lost it and been running without, think about replacing it today. You will save a lot of otherwise wasted gas and cut down on smog.