it used to be movie trailers that ran on tv as commercials, even everyday for 25 times a day for 6 to 8 weeks weeks helped the movie.
for example, " solo a star war story " that today, it just make 158 millions since it was released. it is a big flopper at box office.
two reasons, why it is a big flopper at the box office:
to make only 158 million, the story sucks to watch. possible, too many unknown actors that can't act. I have not see it, or I can wait for dvd awhile to see it.
releasing a second star wars movie too early ( less than six months apart. ) after one was already released in December.
in general, movie trailers usually put out their best scenes in their trailers, and when you see it. something, it was worthy going see the rest of the movie in theater.
another problem with movie trailer, help the movie consider to win the Oscar. not the best picture, but best song. most of the trailers, put their theme songs in it, because sometimes that only reason, why you go to see the movie in the first place.
one of most interest and most movie franchises of all times, are " james bonds " franchise that bond movie is release the trailer will start slowing building to the famous bond theme.
in the movie, most of the bond's theme songs are nominated for best song. some of them , win the Oscars.
interesting, most of movies don't do that only a bond movies do. their movies literally separate at the beginning the song that they want audience to remember first that would nominate first. also recognizing the singer from the movie.
this way , they build up to james bond theme song slowly that are famous for during the movie.