Suspension for speed Here's how it works

Click the image to see the difference between full suspension, and rigid bikes on rough terrain.
Its all about momentum (and inertia) - how an object resists a change of direction.
You want you and your bike to go forward, remain going forward, and fast.
On a rigid bike every hit knocks you up, gravity pull you down, constantly changing you and your bikes direction.
With full suspension (and front suspension to a lesser amount), although the wheels are hammered up and down with the force, its you, the rider that weighs far more than the bike, and holds most the momentum. If your body can maintain a smooth line, your momentum will not be broken, and the only thing slowing you down is you and your brakes. Compare how much the saddle and bars move, it makes a difference.

Climbing with Suspension

Full suspension for climbing?
You're climbing up a steep rooty, rocky trail, you've got your weight forward to get power through the pedals, the rear end hits a bump, knocked into the air, your rear wheel spins, getting no traction, again, you loose your momentum.
Suspension isn't only for jumps, its about keeping you wheels firmly on the ground for as much time as possible. The same hit with rear suspension, the wheel rises, you maintain your forward motion, keeping grip, keeping your momentum going - important for those hard climbs!