Personally I think it’s a bit of third wall breaking by him narrating the story and speaking to you while he tells the story. Part of what makes discworld so funny is the fact that the narrator seems to be in on the joke of it all. And the footnotes. They’re have been some incredible attempts to bring it to the screen but you can’t see inside the person’s head while watching them play the character and a lot of what makes these stories funny are the characters thoughts.