So what's the deal with woodruff keys?

[JohnC]

So, having lost the woodruff key that keeps the crank pulley turning with the crankshaft, I began to ponder... Why is it there? Is it to provide a point of failure between the crankshaft and the accessories? To avoid the potential for damage to the crankshaft (and hence avoid having to dissasemble the engine if the theres an issue with the pulley)... I dunno - Discuss.