For film makers it is vital to make their film known. Especially when that product has a limited shelf life. The most important thing for a film is that it is popular as of its release date, to ensure the film does not suddenly 'disappear' from the media and companies lose out on investments.
Audiences can also be generated from a different media source. For example, the film '50 Shades of Grey' is based on a novel by E.L James
Stars of films may contribute to film marketing, especially if they are associated with a specific genre of film. Audiences who like action films may want to see a movie starring, Jason Statham, because they know that they are going to get a specific sort of action film.
Film distributors may start marketing a film many months before it's release date to create a sense of anticipation from the audience for the release. This can be done using teaser trailers, posters etc, which ends once the film has been released, so films can only be marketed effectively before their release.