(Often with firearms, or firearms of a type, a magazine of a particular size, etc.)
E.g., "Why do you need a 30-round magazine anyway?"
Why do you need access to the Internet? You don't. Yet you have it anyway. Asserting "You do not need X" even if it is true (define "need"; need to live?) is not a justification for stealing or infringing a person's right to obtain or manufacture X. (DR)
My favorite response is, "Why do you need a fire extinguisher?" Obviously, in case of emergency. [HB]