Honestly, I never read The Godfather novels. Seen the films so many times, but I should read the novels shouldn't I?
The GF novel is a unique piece of literature. I read it several times before I saw the film. As with just about any film that's based on a novel, knowledge of the novel's content helps to fill in the blanks for the film's viewers. Also, the back stories that the novel offers can fill out the characters, their motivations, relationships, etc. in the movie.