Hey ZeusGT: The primary use for the six o’clock hold is in competitive bullseye shooting matches when iron/open sights are in use. It is used because it gives a specific, repeatable point of aim, which is more consistent, which makes for better accuracy.
Wanting the bullets to hit in the center of the bullseye to get higher point value, the shooter sights the gun so the rounds hit high, in the center, while aiming at the bottom edge of the bull…..a six o’clock hold.