Like Jad Abumrad said, telling stories orally without the benefit of visuals is ancient. Before we had theater and acted out stories, before we painted on the walls of caves to help tell our tales, we could only speak them, give them words, explain what we meant without being able to show. Rarely have we told stories without sound. Aside from paintings (which usually need cultural context for the story to be understood), the only medium I can think of is silent movies. Even then, there was musical accompaniment and words (visual representations of audible sounds) were written on the screen. Books are just words, just a collection of audible sounds portrayed through a visible medium. Everything goes back to those ancient nights around the campfire, those first stories being told, based in truth with some exaggeration. The first fiction.
With Moon Graffiti, the echoing of sound effects and lack of music shows the isolation and desolation of the moon. They know they are trapped and going to die, and the dramatic amplification of otherwise quiet noises (putting up the flag, snapping the camera) conveys the vast, unending emptiness of space. This story would not work as well in a visual medium, because it would feel like you are there, if that makes sense. It is not two men accepting their fate, it is two men and you, or at least two men and a cameraman. It makes it harder to feel like the story they’re telling is real, like it really happened. Through the audio only, it feels more like we aren’t there, we can’t see their faces, we can only hear their last breaths as they wish they could say goodbye to their families. It makes the story feel more isolated.