Hi Floyd,
A lighter hooklength is used almost as a fuse in an electrical circuit is used - should undue pressure be applied to your set up(perhaps by a fish of extremely large proportions in relation to your kit)then the hooklength will snap before anything else(mainline, rod, arm, etc! ), it will also, and probably more importantly, snap off rather than injure a fish,leaving a small amount of light line attached to the hook rather than yards of heavy mainline which can lead to all sorts of complications for the fish. Obviously lighter hooklength will also be less visible to your quarry than your mainline
Hope this helps.
Wayne