Piping in with some odd history info. They were originally made to give theater patrons something to watch while the main film was re-wound. Theaters used to play the same movie on repeat all day until they got a new film, then it was that movie all day. There were no “move times”. If you showed up midway you just stayed until it repeated so you could catch the beginning. Also why news reels and other visual media would get played.
Psycho made the change to set times. Hitchcock did not want anyone to accidentally see the end without starting from the beginning. Shortly after that those movie trailers got bastardized into the ads we now know them as.