In answering a telephone call, why do you say "Who is this?" instead of "Who is that?"
"This" and "that" are used to reference something. "This" usually refers to something close by. "That" refers to something far away. So the distance differentiates "This" and "That. The distance does not have to be a physical distance but could be a metaphorical distance too; closeness in time or similarity. You can say "Who is this?" to someone on the phone (close to your ear). But you say "Who is that?" to someone on the street while you are sitting in the living room. I hope this clears it up for you. Otherwise, the confusion continues and we don't want that.