There are a number of reasons to buy the Grip. Firstly, its called a grip because it helps add size and well grip. If you have large hands it gives you more room to hold the camera so you don't have to scrunch up your hands as much to hold it. Another reason is for the vertical shutter release and makes it more comfortable to hold vertically if that's something that you do. Added battery life is a bonus along with better balancing with heavier lenses.
In my opinion the weight isn't much compared to the added comfort. And it makes the camera look like a professional set up if that matters to you.