Ever wondered why when they show previews of upcoming movies before that actual movie begins they are called trailers?
When these previews were originally introduced in 1913 they were actually shown after the film ended. After a while they however realized they weren't that effective promoting new upcoming movies as people tend to leave the theatre and not stick around to see the credits or the trailers. So they were moved to be shown before the actual movie begun. But the name stuck and that's why they are still called trailers in these days.
In the past there has also been movies that show bloopers from the making of the movie during, or after the end credits to keep the audience sitting a while longer. I don't know how effective those have been, but apparently that's still something the studios want.
The most effective method for that has probably been the recent invention that especially Marvel uses where they show some previews of their upcoming movies after the end credits. These post-credit scenes should hence be actually called trailers as they are pretty much what the term originally meant.