Why are Teasers Made?

Films may have teaser trailers for any reasons. A teaser trailer is a prequel to the actual trailer of the film to generate a hype about the upcoming films. Recent teaser trailers that I have watched are: The Avengers teaser, The dark Knight Rises teaser and the Men in Black 3 teaser. The reasons these teasers were made are:

• If the film is a sequel to a previous film for example The Dark Knight Rises is a sequel to The Dark Knight
• If the film is an adaptation of a game or a well known novel
• If the film is linked to other films in some way like The Hobbit