Answer:
to keep a third party up-to-date with the subject of the message
Step-by-step explanation:
Courtesy copy, or CC, in the email, is used in order to send the copy of the message to the third-party recipient. In case you have someone else besides the person you are communicating with who can benefit from the information in the e-mail, you can add them under the CC.
This is usually done as a form of supervision or keeping the records of the communication, to make sure all the information is transferred correctly. The unwritten rule is that person in the CC does not reply to the e-mail and does not participate directly in the communication, unless asked to, or when the circumstances demand it.