The term "carbon copy" dates back to the time when Xerox machines were rare and if you wanted to make multiple copies of a document you had to insert carbon paper between pages of typing paper.
The purpose of multiple copies is to send the letter/memo to several different people. For example, you are a department supervisor, and you have written a memo to the company president recommending that an employee gets a promotion. You would send a carbon copy of the memo to the employee so they could have a record of it for their personal file.
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