Why does Raymond Carver use the image of a cathedral as the medium through which the narrator is changed in "Cathedral"?

The image of a cathedral is used after Robert asks the narrator to describe what he sees on a television program. It is significant because it becomes the means to the act of communion between the two men and inspires both of them.
After the narrator's wife falls asleep on the couch, her blind friend Robert, who is visiting her, says he wants to stay up with the narrator. "Something about the church and the Middle Ages was on the TV." The narrator tries other stations, but there is nothing that interests him, so he turns back to the first channel. Robert tells him not to worry about it because he can always learn something new. As he listens, it becomes apparent that Robert cannot envision what a cathedral looks like. He asks his host to describe it to him. The narrator tries to explain it is a medieval church with very high ceilings and spires that took hundreds of years to complete. 
These beautiful churches, built at a time when men were attempting to reach toward the heavens and free themselves from the Dark Ages, were often dedicated to Mary. These cathedrals were huge structures that have spires that reach to the sky, vaulted ceilings, stained glass windows, massive doors carved with Bible stories, and flying buttresses in the Gothic style. The narrator says he cannot really describe these impressive churches. 
Robert asks his host to get some heavy paper and a pen so they can draw together. It is, indeed, significant that the narrator and Robert draw a cathedral, a beautiful structure that attempts to reach to the heavens in supplication to a higher Power. At the time of their construction, cathedrals lifted the hearts and souls of worshipers, as they were inspired by spiritual desires when they saw the beautiful windows with stories of the saints in them. Therefore, the drawing of a cathedral acts as the medium with which the narrator and Robert can both be lifted together and be in communion spiritually. "It was like nothing else in my life up to now" the narrator concludes, as he has finally connected with another person.