Whereas the "main" or "usual" shock is oriented close to the vertical and approximately at 90 deg. to the axle, and is most effective in dampening wheel motion in bump and rebound, the "5th" or auxiliary shock, by virtue of being oriented at about 45 deg. is more effective at dampening chassis roll motion.
If the valving of the main shock were stiff enough to provide the same roll dampening as is provided by the auxiliary shock, it would be too stiff to allow the needed compliance for good tire performance.