What is a trailer?

From my research I have discovered that these elements are often/always incorporated into trailers. This is because these are the aspects of the films that are most likely to appeal to audiences and these elements can also be altered/removed/included to appeal to specific target audiences that the film makers are trying to target. 

They are fairly basic elements meaning that there is a wide scope for how they can be presented which leaves a lot of flexibility for those making the trailer as they can be modified to become specific to the film/target audience. They are also what people generally perceive to be the most important elements of a film as people are more likely to watch a film by a director they know, including actors they know and when they have a rough overview of the story-line/genre of the film.