Most successful directors have a mental image of how a film should unfold.
Directors are, by large, creative, opinionated and strong willed.
When their vision of a movie is in conflict with studio that is footing the bill scenes are often cut. For many years now there have been director's cuts. Releases of films where the director's original vision has been restore. Sometimes, this can diminish a film. Other times it provides great depth to the characters, completes an explanation or even adds previously unseen plot lines.