One way in which corollary discharge helps make sense of behavior is that it helps to coordinate different aspects of behavior. It is important to realize that behavior is not just action potentials sent from one neuron to a motor neuron. Instead behavior is a coordinated circuit of neurons, or motor symphonies. Also a behavior may involve several different motor symphonies. Who is responsible for coordinating these? Corollary discharge. It is one very important way that the nervous system talks to itself.

Corollary discharge is one way the body can detect that there is something wrong with another part of the body. For example, when the body doesn't do what the nervous system tells it to do, the mismatch between corollary discharge and sensory input tells the body there is something wrong. This is why people get motion sickness.

Corollary discharge also explains one reason why the body does not respond to input in the same way every time. In the Pleurobranchea may not withdraw its proboscis when it is eating because it receives corollary discharge signals that tell it that it is eating. So the one sensory input, hitting the proboscis, may result in two behaviors, withdrawing proboscis or not withdrawing.

These are two ways corollary discharge helps to explain behavior we will probably encounter many more.

Actually, you have three. And will encounter more. Don't want to make any guesses/suggestions yourself? PG