The trailer is the main link between the film and getting people to watch the film because if the trailer isn't shown to them they will not want to see the movie; they have never heard of the movie nor have they seen any evidence of what genre the film is and whether or not it looks good enough to pay to watch it.
Also if people do see the trailer and they way it is edited and the camera quality is poor it'll put people off wanting to watch the film; the trailer doesn't do the actual film any justice.
This highlights that the trailer is very important because without one the film wouldn't be exhibited and if the production of the trailer is very poor it'll cause people to not want to go and watch the film.
The main reason of a trailer is to promote the film it is trailering, if it does not do this well enough people will not go and watch the film.
In addition, trailers display the most important parts that relate to the genre off the movie. For instance, if the film is an action film the trailer will display a lot of action and have the dialogue of the movie playing in the background. This is so that people see what they want to see from an action film in the trailer but they also hear some of the main dialogue in the film in the trailer.