Just the other week, I was watching the commentary/documentary on the making of Milan Kundera's "Unbearable Lightness of Being". The question of "why read the book" is raised there, in a sense. I was amazed to hear the director speak of Milan Kundera's open-minded attitude. Kundera said "Do whatever you feel necessary to make the movie, and take liberties with the story" (paraphrased). I thought for sure Kundera would be more rigid on that issue, since he is so particular about his translators. Apparently Kundera had worked with screen plays and movie production, and even taught a course in cinematography, so he understands that a movie cannot and should not always be a carbon copy of a book. The documentary describes the movie as a supplement or enhancement to the experience of reading the novel, rather than a substitute for reading it.
One of my favorite scenes in the novel is at the end, when they startle the moth and it slowly circles the room. The director said that they filmed that scene, and several others, as a potential ending. I realized that I should not be disappointed that the movie has a slightly different ending from the novel.
I found that reading the novel enhanced my enjoyment of the movie, and watching the move added to my understanding of the novel.
By contrast, the television production of Brideshead Revisited was an exact copy of the novel by Evelyn Waugh. I can turn to any page in the novel and remember hearing the dialogue in the movie. I also found that the movie and book compliment each other.
I remember the documentary on "The Shawshank Redemption", which observed that, in the novel, the character of Red was only a page or less, but in the movie, Red was developed into a major character.
Stephen Mallerme once said that a poem is never finished but only abandoned.
In some sense, a novel need not be finished, but may be enhanced by the creativity of a movie director.
Certainly, the story of the Gospels has been "enhanced" over the centuries by people like Kozanzakis and musicals like the Superstar, as have the ancient Greek myths, by writers like Camus with his Sisyphus.
Sometimes, watching movie version can provide motivation for tackling the novel. I know that watching Moby Dick did that for me.