The boxing bag practice is important for creating strength and speed. In any fight, or match, you will need this speed to win over your opponent. It may only take 10 to 15 seconds to throw off your opponent and give them a solid warning that you can defend yourself. In a match the speed can make your opponent less likely to jump in and go for a knock out.
The practice is highly cardio in nature. It won't take long and you find yourself sweating which is greater for losing weight. This is one of the reasons why boxers are in such great shape. It is these bursts of energy that allow for greater fat burning potential.