Why would Miller include the note on historical accuracy?

Miller's intent was to inform the audience that although his story is fictional, it is also based on the truth. He makes sure his audience understands that his characters are fictional, but that each of the characters brings a bit of reality to the stage. By doing this, Miller purposefully reduced the chances he'd be accused of libel and misrepresenting the truth, and in doing so also opened the door to being as accurate as possible in the story he depicted.