The real reason that movie previews are called trailers is that the industry is just too set in its ways at this point. The longer explanation however goes all the way back to 1913 when people could pay their admission to a movie house and then sit there pretty much all day waiting to see whatever was playing. It could have been just about anything, but so long as they paid their admission they could stay. Trying that these days would get a person booted out pretty quickly since admission is only ever for one movie. But back then a movie theater advertising manager named Nils Granlund came up with the brilliant idea of adding on rehearsal footage of the next upcoming attractions. This upped the interest of the average viewer and made it more of a certainty that they’d be back to see the new attractions that were coming out. That was essentially the start of the movie trailer, since back then it made sense to call it such as it came after the film.
Producer William Selig decided to take it one step further when he brought the popular serial format to the big screen, making it so that people could come in, sit down and enjoy a tale of adventure or drama, or whatever, and then catch a preview at the end, a trailer, of what was to come next. The serial format allowed people to get just a glimpse of what might happen in the next episode and would thereby pique their interest and likely bring them back for more. That idea continued to grow and grow until finally the serial format was driven out of movies and the previews started to come before the films, keeping the name of ‘trailer’ since it was already in use was was easier than naming them something else.
To this day trailers are watched almost compulsively by movie buffs, sometimes to see what’s coming out and sometimes to try and make educated guesses on what kind of material the film is going to present. Some folks even like the trailers more than they like the movie since the precious balance that’s given over to a trailer is sometimes disregarded and gives away too much of the movie before the people end up seeing it. The trick of any trailer is to make the people interested and give them just enough that they’ll be rushing to the theater to see what really happens in the moment the screen cuts away or when the bad guy is about to face the hero and so on and so forth. These days however some trailers give away far too much and kind of ruin the idea of the movie before people ever get there.
Plus, with some movies there is an endless line of trailers that go on for so long that it’s possible for people to forget what they came to watch. The trailers are great, but sometimes watching them on YouTube is better since you can move from one to the next without having to watch them all.