At the beginning of the 20th century, first movies were mostly even without any words, so music was like an emotional guide for an audience.
It makes it easier for the movie director to let you feel the way he wants you to feel during specific scenes.
For an example that one song in back to the future, just a short sound that makes a shiver crawling up and down my spine as I well expect something interesting to happen.
Sometimes the music does not only guide as an emotional red line through the movie but it also can totally take the lead and describe the whole movie. Mostly artistic movies with less or no conversation. So years after knowing a movie, just listening to the soundtrack and or sound score makes you realize certain scenes from any movie.
Music and theatre
It is kinda similar to how music works in our lives. Explicit music will make us remember specific parts of our life.
Music has such power. But since movies came a long way, now sharing the world with opera, musicals and theater, I think music will stay important.