Why is sound so important in a film?

Gorge Lukas (Director of the Star Wars Franchise) says that "sound is 50% [of the film]" and I agree whole hartedly.
Sound is important in a film because it's what you hear. Thus giving you context to the film, dialogue between characters, the noise and bustle of the location, the soundtrack chosen, the sound effects, it can even give away the genre.
When I looked at a movie trailer with no sound I found it to be extremely boring, it wasn't engaging to me and I was confused most of the time. However when I looked at it with sound, the entire mood changed, the dialogue captured me and got me interested in watching the rest of the film, the music was good too.