The existence of varying public manifestations of a work of fiction liberates the viewers (or readers) vision of the work again from the oppressive feeling that a single representation might have on him.
That means if you read a book, you make your own private interpretation of it.. Then you see the movie and in most cases people therafter might not be able to enyoj the book again as they did before because the movie will cloud their fantasy. But if there are several films about the book this will most likely not happen. Or at least the reader has the choice of wich aspects of which movie he will integrate into his vision. And most of the time it won't be the aspects that had the most budget dediated to special effects...
...at least this is my humble personal experience.