An important element of sound design is mixing. The audio mix is an important element in realism and it also functions to set tone and mood. The audio mix determines the relative volume and levels of every audio element in the film.
If the film is striving for realism, then a good sound mix will be absolutely necessary. No sound can be too loud or too quiet. All of the dialogue will need to be at the perfect level.
For a horror movie, the sound mix might play a part in the jump scares, or a low-pitched sound mix can contribute to the eerie tone of the film. If making an experimental film, the sound design is part of the experimental nature. In that case, there might not be a need for perfect sound mix.
The mix is usually the last element of the sound design process, but it can be just as important as all other elements.