Audio is a crucial part of a film, without it, you just have a picture of something moving on screen without context or any clue as to what is going on. Sound gives us the other part of the story, it tells us what is happening on screen.
Sound design can add to the narrative of the film by satisfying the ear of the viewer and give hints as to what is occurring on-screen to assist the viewer in placing themselves in the setting of the documentary. Things such as atmos that really matches what the setting is on the screen, can help place the viewer in the world of the documentary. Sound effects montages can help give clues as to what line the story is about to take.
For most people, it is easier to digest information audibly rather than having to decode what is happening on screen. This is especially true for the documentary. Great sound design can make a film more interesting to the viewer of the film, but really all it needs to be is non-distracting to keep the listener sucked into the film. What this means is that bad sound can ruin your film but good sound will go unnoticed and suck the viewer in. For instance in a documentary where you have an extremely emotionally charged moment in the film, but the narrator's voice is towering over the rest of the soundscape. The viewer will be highly distracted by the towering volume of the narrator and thus the film will be ruined. But if the levels all match up and the transitions between sound are good, then the soundtrack will become second to what is happening on the screen, and in turn supporting what is happening with the visual aspect of the documentary.
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