It's for Piercing Howl. It's great for kiting stuff, and also a great way to round up streaming adds (Dazed mobs bunch up with the undazed mobs).
Xav of Premonition has a video floating around somewhere where he does the LDW hard mode add tanking by kiting them all over the room so the other tanks don't have to deal with it. There are other applications for it. I used to use it for Heroic Saurfang to get a quick slow off on all the creepy crawlies as soon as they spawn. You can also use it on Lich King for getting a slow off on the Valks.