When you need to keep the original document or send the document to multiple people.
Explanation:
If you need to keep the original document (eg. marriage documents or birth certificate or something important), you would make a copy of it (unless you are sending it to a lawyer or a government agency).
If you, for instance, have a memo for your company that all your employees need to read, you would copy the document so you would be able to print it and send it to everyone without passing the document one at a time.
