It is important to do it after your weight training because its purpose is different than traditional cardio. It does improve cardiovascular fitness and endurance but the main purpose is to burn off any remaining stores of glycogen that could prevent you from burning as much fat as possible. It is equally important to perform your resistance exercises or weight training when you have the maximum amount of energy and strength required to grow and tone muscle. If you do this after your cardio, you'll miss out on the maximum benefit possible through resistance training.
