The first thing you will notice is that he isn't standing still, he is ducking, bobbing and weaving, your punches just land into thin air. At this point you start the real training process. If you are thanking God that the session is over, you have never worked so hard in your life, past all your limits, then you have a good boxing trainer. It's bad habits that will kill you, a trainer picks up on these things and corrects them. A trainer will make you do the things you don't want to do.
Trainers seem to disagree with each other from proper handwraps to almost every training technique.
It would be boring if everybody agreed, if all fighters had exact same style. There are several right ways to accomplish the same thing. I like to tell people this is because they got hit in the head too much.