Two reasons.
1. It doubles your battery life. The camera uses up the juice in the grip batteries first, then switches over to the battery pack in the camera body itself.
2. It gives you double the available area to hold on to, and it allows you to shoot vertically without twisting your hand/wrist/arm around.
Plus, it looks cool. It makes your camera look like a D4!
One of the things I have been impressed with is the battery life of Nikon batteries. But the holding area appears to be a positive benefit, although it makes it that bit more difficult to stow away in a camera bag!