When you are at the gas station filling your gas tank and the nozzle automatically clicks off, this means that your tank is full and you should not add more. The pump is designed to turn off when the fuel in your tank reaches a certain level. This level leaves room in the tank for expansion and to allow the evaporative emissions system of your vehicle to operate properly. The evaporative emission system of your vehicle is designed to collect and re-burn fuel vapors to reduce harmful emissions from our vehicles.
When that air dome is reduced from topping off your tank, after the pump automatically clicks off, raw gasoline can enter into an important component of your vehicle's evaporative emissions system, the carbon filter aka the charcoal canister, this component is designed to carry only gas vapors. This raw fuel can foul the evaporative emission system, causing it to not work properly.
When raw gasoline enters the filter/canister, it can either spill on the ground or it can cause issues with engine operation, causing poor fuel mileage, poor emissions and overall decreased engine performance.
Gasoline vapors contribute to bad ozone days and are a source of toxic air pollutants such as benzene. The gas station pumps are equipped with vapor recovery systems that feed back gas vapors into their tanks to prevent vapors from escaping into the air and adding to pollution. So if you overfill your tank the extra fuel may be drawn back into the vapor line and fed back into the station's storage tanks.
So not topping off your tank will help keep the environment clean, and will save you money at the gas station.