The tapered thread on black pipe, etc. allows for a greater surface area to be in contact between the pipe and fitting. B.I. pipe comes with a "thread protector" on one end that is straight thread but looks like a coupling. If they're used, they usually leak like crazy. (A plumber I know had to redo the gas on 5 complete houses when the guy he sent out came back with a whole box of unused couplings on his truck)
