As noted in "What's Up With the Ending?" Carver insisted that there is nothing symbolic about the two men drawing a cathedral. It isn't "a metaphor for art" (source). Some readers might find this enormously comforting. Others might note that the drawing of the cathedral can't be a metaphor for art, because it is art. The two men are making art. The process of creating this art seems to come in three phases. The first phase is exemplified by this quote:
How could I even begin to describe [a cathedral]? But say my life depended on it. Say my life was being threatened by an insane guy who said I had to do it or else. (3.10)
The narrator is reaching to express what he sees by creating something that looks like it, out of words. For an artist to create art, there is usually a trace of this urgency. That's part of what motivates the artist to create. In any case, the narrator keeps trying and trying but he just can't do it. He can't find the right words for the task. He's completely frustrated. But, when Robert suggests drawing, the frustration gives way to fluidity.
In the final stage, when the narrator closes his eyes, he loses himself even more deeply. Notice that none of this comments on the quality of the drawing. The readers can't judge the actual piece of art created. They can only judge the narrator's description of the experience of creating it. When we see how much he needs the experience, the narrator's words resonate. When we consider the possibility that his wife is headed for another suicide attempt, the words take on ever deeper significance.