In auteurist theory the film director is seen as the film's author, the one responsible for its quality and vision. This was important because it meant that critics could analyse films rather like a book, like a work of art. Alternatively, others argue that films are commercial products rather than works of art. In addition, they are the result of the efforts of a whole host of people: actors, cameramen, editors etc. rather than one person's work.
Audiences who go to see a film because of its director will respond differently to those who go to see a film because of its star. They are more likely to attempt to unpick its meaning, identify aspects of the director's style.