I like to think of it as a pattern -- something that has been proven as one way to tell a story. Screenplays are particularly conducive to this patterning because (1) movies are generally about 90-120 minutes so not a huge variation in length, (2) each page of the screenplay is about a minute on screen so you can easily project where critical points usually sit, e.g., the inciting incident, and (3) really big budget films seem to be made more on how not to lose money rather than how to try something original and potentially misfire. The third point is why Hollywood prefers making a movie with a proven market and audience (i.e., proven book sales) than using original spec screenplays.