The letter shows that Marquis actually knew nothing about Lincoln except that he was president. As history tells us Mary Todd had some serious mental issues and in reality, Abe was at his wits-end with dealing with her. It was not a good relationship. So, Abe probably would not have included the "Ole Mary Todd is waiting" line. Clearly this is a forgery. Also, the letter says nothing. It’s just a bunch of platitudes, which makes it particularly funny when you think about Marquis composing it and then his watching the tearful John Ruth read it. Finally, the letter is used at two of the funniest/darkest parts of the movie. The first time, where John Ruth reads it, and it ends "with a punch line". Marquis punches Daisy in the mouth. Then the final scene, with the two former, ”natural-enemies", covered in blood and gore, nuts shot off, and dying together in the bed. Patriotic music plays in the background and the camera slowly pans up behind a severed arm dangling from a hung dead woman. "That's a nice touch" is the punch line for this scene and for the dark humor running through the entire movie. Once you get the joke, "that's a nice touch" gets funnier with every viewing.