Why is sound so important in film ?

Firstly, sound is the Oral element of the film (what we can hear)

Sound in film is extremely important. For example sound is used in film to establish tone, atmosphere and to help give the audience clues about the plot,  characters or to identify the Genre about the film. It has also been used to help something called 'suspended belief' to create realism for the viewer and heighten their experience of the film.
George Lukas has previously said that sound is 50%  of the film, alternatively saying that without the sound the film would be half as good.

Sound techniques are often used to convey a certain mood of the film and manipulates the audiences reaction. This is particularly true in thrillers and horror films, which both seek to create a sense of fear. Most horror films rely on the jump scare technique to invoke that all important scream- worthy moment that makes a horror movie a success.

The sound effects you hear in any given scene are often not the sounds that where recored in production. Most sound effects, such as car horns, footsteps, and gunshots are recreated and added in later by sound editorial.  This makes the sound much more realistic an gives the audience a sense of realism.