I know Bitcoin is being used to buy illegal items such as drugs and other stuff. I think they use Bitcoin because it's what the dealers and consumers want, anonymity, untraceable, and still be transparent. I don't know any deep web stuff but I think it really is being used not just for drugs, I've seen in Mr. Robot that it is being used in other ways. (It is a good show by the way). Drugs are just going to destroy you.