Correlary discharge helps to make better sense of behvior. When a neuron is activated, before it carries out it's specific pattern and function, it sends another message to other areas of the brain to have a copy of the command"on file". It makes it possible for the body to have have a back-up copy or a "double check" system, to make sure that the proper action or system of actions was activated. Motion sickness helps to clarify this; the muscles of our body aren't registering any movement so our brain doesn't get any messages telling us that we are moving, yet our inner ear senses the swaying and we become ill. It makes sense that a back up system to monitor our intended activities (neuron activity) and the outcome of our neurons activity, considering the vast responsibilities that our brain must carry out.
Its not so much a "backup" system, in the sense of keeping a copy of the command stored somewhere in case its needed again. Better to think of it as notification by one part of the brain to another that the first has done something, and the second may want to do something differently because of that, or should expect something because of it (like an input from the inner ear). That make sense? PG