Given that most airlines are charging for checked luggage, you could save up to $30 per trip if you pack a carry-on instead. Airlines charge for checked bags because additional fuel is used to transport additional weight. Packing lightly is an easy way to conserve fuel, which reduces greenhouse gas emissions.
Traveling with just a carry-on also saves you time—eliminating the minutes spent at both check-in and at baggage claim. This can potentially give you an extra hour on either side of your flight.