The Star System

I think the star system was particularly important to how classical Hollywood functioned. How it worked kind of reminds me of sport franchises today. The Dallas Cowboys got Tony Romo and associated him with their brand and whenever you hear his name you think of the Cowboys. Similarly, the movie studios tied their fortunes with the abilities of their stars. When their stars were well known and performed well, the studio made money. People went to the movies to see their favorite actor or actress. Since those stars were tied to a studio, this translated into those people going to see the studio's movies.
Since the stars were so important, the movies had to be tailored to the stars' strengths. The movies were written in such a way that the stars were front and center at all times. while it might be kind of odd for people to dance and sing in a movie, if Judy Garland was good at dancing and singing, dance and sing they would. Also, things would tend to repeat themselves. A star was discovered to be good at playing a certain type of character such as Bogart's hardboiled detective, and would play that same character over and over again, with only the name changing. The typecasting of stars still happens today and is really obvious in the case of actors like Michael Cera or Will Ferrell.
The Wizard of Oz is a good example in which a star dominated the movie. It was originally adopted from a book and was changed quite a bit for Judy Garland. All those musical numbers weren't quite there in the original story, but Garland could sing beautifully, so they were added in to fit the movie to her talents. Also as was brought in lecture, not many people know the director or most of the supporting cast, but everyone knows the star, and that's just how the studio wanted it.