Many people will tell you that it's for safety and all that, but really, originally, it had to do more with ease of construction and reliability. For a very long time, blades - particularly the tangs - were filed to match hardened patterns. It's a heck of a lot easier to file a square tang profile (the half stop is the flat end of the tang) than a rounded one (actually the profile isn't quite square, but we'll ignore that for now). Also, for a long time the old mark of quality construction was the "clean and square" joint - basically the square tang was matched to a square bolster, essentially filling the joint, even when closed. This meant that if you dropped your knife in the dirt, the dirt didn't get into and around the joint to foul up the mechanism. (Remington "discovered" this early last century.
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A square tang is largely self cleaning -- the corners sweep debris either toward the end of the knife or back into the body - either way, the debris gets moved out from under the tang/joint mechanism where it can then be removed. Properly cleaning up an old, gunked up knife and getting all the crud out of the mechanism is a lot easier with one that has square tangs.
Also, a square tang causes less wear and tear on the spring over time - assuming the corners of the square tang were rounded a tiny bit in construction. The corners sweep back and forth along a length of the spring in operation. spreading the wear evenly along that length. A round or cam end tang concentrates the wear in a much smaller area of the spring - becoming a problem when proper maintenance and oiling is neglected.
I'm a fan of a good, crisp half stop, myself. :thumbup:
