Historically, seals have been used on letters to verify the authenticity of the sender. I always firgured you’d have to be pretty wealthy to have your own seal for letters. Especially since seals would be hard to transport.
I guess it is pretty convincing though. Especially if it is a specific seal. I know if I got a letter with the president’s seal on it, I’d be like, “I’m convinced this is from the president. Thank you for the letter Mr. President.”