The purpose of Punch bag training is not just to hit as hard as you possibly can, but there is a lot more to it. If you want to get better, then push yourself beyond limits. That is why this kind of training should be done in front of a trained coach or trainer, as he will not let you slack.
How does professional boxers and MMA fighters go about their daily heavy bag training?
How does professional boxers and MMA fighters go about their daily heavy bag training?