The purposes of a trailor

There are a few reasons why trailors are used.
Trailors are used for advertisement. It gets the film out in public so people will hear about it through television, cinema or even the internet.
Its also to attract the target audience and draw them into watching the film by making the film look as attractive as possible.
Trailors are used as a teaser as well.  To give the audience a taster of the film and ultimately try to get people to want to see the film.