Answer:
Because that creates suspense and helps to make the audience feel more thrilled when they see the real trick. You see... the fake trick makes the audience this is it. Like.. this is what the guy is capable of but that is not the truth. So when he performs the real trick, the amazement of the people will be more than the amazement they would have if they had seen the real trick just like that. So.. yeah. It helped to bring more excitement.
