What is the significance of Carver's choice of a cathedral as catalyst for the narrator's learning experience, and what dimension does it bring?

It isn't explicitly shown in the story, but the cathedral suggests a religious or spiritual experience. The cathedral symbolizes the themes of the story. Change and transformation can occur in a religious, holy place such as a cathedral, and it is a place where humans can connect with God and with each other. The major themes of the story deal with the alienation and loneliness of the narrator and his wife. Through the experience with Robert, it's suggested that both the narrator and his wife have a chance of perhaps connecting with one another and with other people by the end of the story. The world opens up for the narrator once he closes his eyes and imagines the cathedral, suggesting perhaps a transformation in his life, even if it is a small one. In order to reach that point, the narrator must use his imagination and creativity, suggesting the power that imagination can have in helping people to overcome difficulties in their lives. Many times, a person can experience an epiphany, or great insight, in a religious service, and the narrator has such an epiphany at the very end when he's able to "see" the cathedral with his eyes closed.