Sometimes it elevates the scene to a new level.
Remember the iconic Avengers shot?
In the trailer it has a different more generic soundtrack and it doesn't feel as good.
There are videos on YouTube which re edited Justice League scenes and added better Hanz Zimmer soundtracks to it and the scenes completely change.
OST can make a movie or break it at times.
Obviously other things also matter and all these things need to mesh well together in order to make a good movie.