A customer is emailing an issue and they are sending it to you directly and copying email@example.com. Which creates a ticket. You then reply to the customer and leave the copy in your email for firstname.lastname@example.org which then creates a second ticket.
HDM should be smart enough to read the subject line and understand that the reply is part of the original ticket and append to the ticket rather than creating a new ticket. Not everyone will use the 'add note' feature.