We learn language by trying them out in production. When we attempt to transmit a message, we fail, try again, and eventually arrive at the correct form of our utterance, our partner understands, and we acquire the new form. When a communication is successful, our conscious hypothesis about the rule or item is confirmed. On the other hand, if we experience communicative failure, or correction, our hypothesis is disconfirmed, and we alter it.
Merrill Swain does not claim that CO is responsible for all or most of our language competence. Rather, the claim is that output facilitates learning in ways that are different from input or enhance it.