This is the theme in "Cathedral" that pulls everybody in. We all want to be better communicators. We all have trouble, at least from time to time, expressing ourselves, and understanding what those around us are trying to say. Ironically, miscommunication, misinterpretation, and difficulty of expression usually happen when we're trying to say important things. Sometimes we can't even express important things to ourselves, because we are confused. Even if we aren't, the other person might get defensive, or angry, or simply not understand. "Cathedral" suggests that if we get creative, and try to stand in each other's shoes, there is hope for positive and clear communication even in seemingly hopeless scenarios.
It isn't important that Robert has a better idea of a cathedral, only that the narrator tries to connect with him by sharing something he can see with Robert.
"Cathedral" emphasizes the importance of physical contact in communication.