Traditional camber provides stability and edge grip, especially on hard-packed snow. Racers, hard-charging big mountain skiers benefit from camber at high speeds and variable snow conditions. Many touring skis are traditional camber or rockered just in the shovel, as most of your skin traction comes from the back 2/3's of the ski.
The predeccesor of rocker design was known as tip-splay, and was first incorporated into Super G race skis in order to ease turn initiation. Effectivly bringing the contact point closer to the skier makes it easier to pivot the ski, and is why some beginner model skis incorporate slight tip and tail rocker (K2 calls this 'catch-free rocker'). Rocker shape also helps a ski to plane in the deeper snow, allowing you to stay in a more centered stance. When you don't have to lean back to keep your tips up, you can shred pow a lot faster and longer with less fatigue. Rocker also helps to plow through/over crud. It's kind of like driving over a speedbump with 40inch wheels instead of 17's, much smoother. Rocker in a park application makes butter's more buttery, smoother and catch-free.
My every-day skis are the Armada JJ's, which have the best of both worlds. Traditional camber and sidecut underfoot for good stability and grip, and rocker tip and tail for float and crud busting abilities. Float like a butterfly, sting like a bee. Sometimes on older, steeper skin tracks I get some slippage because of the rockered tail. But for me, its all about the ride down!