The battery grip gives you the option of :
- extended battery life with additional battery
- better grip in a way especially with portrait orientation shooting
- possibly giving a better balance especially on longer heavier lenses
- gives u option of using 6 AA batteries if somehow ur original Nikon batts ran out of juice..
* on certain models, they can give u faster fps
You have to see if u really need the additional battery grip that will bulk up things, else really can do without it with ur existing setup. A needless battery grip kinda of defeats the purpose of having a smaller and lighter camera
Ryan