The difference in a nut shell is that hydraulic shocks actually dampen vibration and absorb the shock.
Spring shocks without motion dampes are prone to vibrations, oscilations, and will litterally destroy themselves. Springs shocks without motion dampers can be down right dangerous on any serious vehicle, because they'll start vibrating and dribbling the tire like a basket ball. Once that happens, it about like driving on very rough ice. And I bet you can guess what happens next...