There are TWO uses for the Warlock Punch.
1. Punish a Broken shield. Ganondorf does have a fair shot at breaking a shield with his powerful aerials and stuff. So if that happens, you know the drill.
2. Recovery. A reverse reverse Warlock Punch has the property of very good aerial control not to mention the momentum boost when Ganondorf does his backhand, so you can charge up a Murder Fist while gaining ground towards the stage if you DIed a hit right in the first place.
In every other situation you would consider the Warlock Punch, you should use the U-tilt however. IIRC the U-tilt comes out faster. It has a LOT more range, has a LOT less ending lag, and does more shield damage. And that wind effect thing helps you land the attack, it can gimp Ike's Side-B, detonate mines, uh yeah. And best of all, your opponent still looks like a nub if they're hit by this attack.