What function does the Star system play in Hollywood cinema?

If this were the case why is Casablanca remembered for the presence of Humphrey Bogart and Ingrid Bergman and not as "A Michael Curtiz Film" or "It's a Wonderful Life" not for Frank Capra's influence more than for Jimmy Stewart's presence. Indeed even with a big name director such as Howard Hawks, The Big Sleep is memorable to most for Bogart and Lauren Baccall's on-screen chemistry and Bringing Up Baby is known as a screwball comedy starring Cary Grant. Is this simply a result of the studio's conscious marketing of their players as such or does an actor or star bring something tangible to a film as important as the director's choice of lenses or camera angles?.
             Before 1908 to 1909, films bore no reference to their players, either in marketing material or in the form of a credits reel. Actors did not even appear frequently enough to be recognised. By 1910, however star names began to appear on film marketing literature and two stars where exclusively identified with the studios that produced the films in which they appeared. Florence Lawrence had become "The Biograph Girl" and Florence Turner "The Vitagraph Girl" and became a useful marketing tool. How did this shift in the respect paid to film actors occur? Many attribute this to a rise in audience interest, with viewer's asking theatre managers for actor's names or studios for pictures and autographs. It is often written that independent producers such as Carl Laemlle then encouraged this as a method to gain strength within the market to battle the MPPC and its monopolistic practices. This very straight forward history is opposed by more modern film historians, who cite changes in the theatre as being influential too. Either way after the release of Griffith's "Lady Escapade" a critic was quoted as writing: "the chief honours of the of the picture are borne by the now famous Biograph Girl .This lady combines with very great personal attributes very fine dramatic abilities indeed.