Why do we have an introduction in the short story "RIP Van Winkle"?

"Rip Van Winkle" is a frame story that begins with a commentary from an unnamed writer. Before the start of the story, there are three paragraphs in brackets, which explain the story's origin. The story was discovered "among the papers of the late Diedrich Knickerbocker," a man who spent his life studying and recording the history of the Dutch inhabitants of upstate New York. Knickerbocker's published history, the narrator claims, is known for its "scrupulous accuracy," and the tale of "Rip Van Winkle," therefore, should be accepted as truth.