Training is important to prepare yourself for the stress situation of a cardiac arrest. Besides information what to do or not to do, skills have to be trained.
Training can be done in different ways, different intervals and on different manikins. But in the end the victim will not be a manikin you have trained on.
Optimally the rescuer is trained with a CPR-feedback (just like an AED) and should use this feedback during the resuscitation itself.