Mr Ken - when you cut a deep line, you push the graver through the case and actually cut the softer steel below while the case breaks away in small pieces. Since the case is broken, as opposed to cut, there are small, raggedy bits (burrs) left attached. A fine line is an attempt to cut the case layer and it is quite hard. The hard case layer tends to break the graver point.
I think the case layer may add a very small amount of strength to tha action, but wear and corrosion resistance are its real forte.
