Why are the addresses of argc and argv 12 bytes apart?

On your system, the first few integer or pointer arguments are passed in registers and have no addresses. When you take their addresses with &argc or &argv, the compiler has to fabricate addresses by writing the register contents to stack locations and giving you the addresses of those stack locations. In doing so, the compiler chooses, in a sense, whatever stack locations happen to be convenient for it.

Why are the addresses of argc and argv 12 bytes apart?

From the perspective of the language standard, the answer is "no particular reason". C does not specify or imply any relationship between the addresses of function parameters. @EricPostpischil describes what is probably happening in your particular implementation, but those details would be different for an implementation in which all arguments are passed on the stack, and that is not the only alternative.

Moreover, I'm having trouble coming up with a way in which such information could be useful within a program. For example, even if you "know" that the address of argv is 12 bytes before the address of argc, there's still no defined way to compute one of those pointers from the other.