Why are trailers called "trailers"? Considering the verb "to trail" means "to follow behind", why would something that is shown before the main movie be called a trailer?
As it happens, Priceonomics has already investigated this:
Built in the early 1900s, the United States’ first permanent movie theaters featured only one screen. Things worked a bit differently back then: you’d pay your nickel, take a seat, and watch a continuous loop of a feature (mixed with cartoon interludes) for as long as your heart desired.
In 1912, Broadway producer Nils Granlund was hired as the advertising manager for a chain of East Coast movie theaters owned by entrepreneur Marcus Loew. Here, Granlund produced the first-ever trailer -- a one-minute spot for the upcoming Broadway show, The Pleasure Seekers -- which featured mainly cut up clips from the production’s rehearsals. The advertisement was shown to audiences after the feature film, rather than before it.
That's for a Broadway show, but movies followed:
That same year, in Chicago, an early film visionary by the name of William Selig decided to apply 19th century society’s interest in print serials (stories published in installments) to films. What resulted was a 13-episode film serial called “The Adventures of Kathlyn.” As with print serials, it was important for Selig to end each segment on a suspenseful note, so as to encourage people to come see the next one. To do this, he decided to include a brief teaser of "next week's" installment at the end of each one.
But when did they start appearing before the main movie?
Most film historians contend that at some point in the late 1930s, theaters began showing movie trailers before the feature film rather than afterward -- most likely because serial-style films were on their way out, and patrons often left the theater immediately following the film. Showing the trailers before, while the audience was obviously captive, proved more effective.
And yes, people have tried renaming them:
“[Studios] have tried calling them ‘previews,’ or ‘prevues of coming attractions,’” reasoned a Paramount producer in an interview-- but society has, so far, refused to accept anything other than what it’s used to.
Go read the whole article, it is fascinating.
They are meant to be following the end of the previous movie until the next movie begins.