This question was inspired by this question, which asks about the history of one work claiming itself as an adaptation or abridgment of another. In The Princess Bride by William Goldman, the author claims that it is an abridgment of a work by the non-existent S. Morgenstern. Why does he do this? What effect does it have on the understanding of the work?
The abridgment is part of the overall frame story. The frame story is very different in the book than in the film.
The book's frame story is very cynical. It's about the disillusionment of children growing up, and the business of fairy tales. The "abridgment" is one element of that: his realization that the story that was read to him was not, in fact, the actual story. The actual story was long and boring. His "father" concocted a fairy tale out of a supposedly true story.
The fictional "Goldman" (who is biographically different from the real Goldman, who has two daughters rather than one son) is a rather bitter person, and that's parallel to his realization that the book is so different from what he remembers. He's also cynical about love: in the frame story, he attempts to cheat on his wife (and is shot down). The "true love" of Buttercup and Westley contrasts with the reality of "Goldman's" love life.
It's rather remarkable that the movie excised this element and took the inner fairy tale at face value, completely rewriting the frame story. Such a change would seem to have a low chance of working, and yet it's arguably a considerable improvement on the book. The book's cynicism verges on misogyny (or "stomps right into"), and that does not bear the test of time well. The film doesn't exactly pass the Bechdel test, either, but it makes an uplifting story out of one that's fundamentally a downer. That's pretty amazing.