The narrator in the short story "Cathedral" is a very cynical, judgmental, bitter man who can't find happiness or satisfaction in anyone or anything around him. He is not happy about the blind man, Robert, coming to visit, and expresses a lot of sardonic displeasure at his wife's relationship with him. He is also nervous about the visit; he says, "He was no one I knew. And his being blind bothered me." He doesn't know how he will entertain the man, or what they will talk about.
So, before Robert even gets there, the narrator is drinking--probably because he is nervous, and wants to relax a bit. Immediately upon Robert's arrival, the narrator offers him a drink, and mentions that drinking is "a bit of a pastime" for him. Possibly, the narrator is a drinker. They settle in and pretty quickly have several drinks. This helps to loosen up the narrator a bit; it puts him at ease, and makes him less nervous about Robert's visit. After dinner they break out the weed, and that relaxes the narrator even more. Normally an uptight and picky man, he becomes easy-going and comfortable with Robert. This enables him to be open to the experience that ends up impacting him so greatly, the drawing of the cathedral with his eyes closed. If he hadn't been relaxed and eased through drinking and smoking, he might not have consented to letting a man grab his hand and guide him through drawing a cathedral. The drinking and smoking ease his stress and strain, and through the drawing of the cathedral, he has a very profound experience. For the first time in his life, he is able to see something from someone else's perspective. It is "like nothing I've experienced" in his life before, and seems to floor and awe the rather closed and un-impressible narrator.
Now, the narrator might have been open to different experiences even if he hadn't been smoking or drinking--who knows. But, it is that that puts him at ease, keeps him socializing with Robert even after his wife went to bed, and lended an air of relaxed comraderie between the two men. I hope that helps; good luck!