Sounds nefarious. Why would a sender want the recipient to verify authenticity while being able to deny it to a 3rd party. Sending threats or blackmail come to

Sounds nefarious. Why would a sender want the recipient to verify authenticity while being able to deny it to a 3rd party. Sending threats or blackmail come to mind as top uses. Breaking confidentiality agreements is another. What's a legit honest above board use?