Examples: Itchy & Scratchy from The Simpsons, The Scary Door from Futurama, or The Grand Inquisitor from Dostoevsky.
The Princess Bride is one of my favorite examples of this, especially because the “story within the story” is the main story, which is unusual.
When I was a kid I absolutely loved movies with this format. It was like I was learning the story along with the characters on screen, and it just made it feel more real. Like the story was so old and with enough truth to it that they made a movie just about people learning about said story. It let you feel like the caring, kind old narrator was your adoptive grandpa, and he was revealing to you some ancient, fantastical part of our history. One that you could imagine really happened, even if the story had some exaggerations. Those opening sequences where they show a big old, leather bound book opening up to the first chapter (e.g. The Sword in the Stone)? HOOK IT TO MY VEINS
