The number of reasons for a grip are many: Giving the same stability in vertical orientation as in horizontal orientation...look where your right arm is shooting a portrait...over your head in a very unstable position. You can gain a stop or more of shutter speed options by being able to shoot vertically as steady as horizontally because both would be done by tucking in arms tightly against the chest when using a grip. Normally you need to increase shutter speed in vertical orientation to reduce the impact of the worse right arm position without the grip. For available light portrait shooters a grip is essential
Less hand fatigue. Shooting for long periods with the small grip area on any camera other than D2, D3 or D4 which have built-n grips. That because with less finger contact area, a firm hold means high finger tension on the 3 fingers supporting the camera on the right side. By having more contact area using the extended grip area, less tension is maintained by all the fingers. It also frees up the index finger for command wheel and shutter functions without increasing the gripping force on those fingers remaining on the grip. After shooting for 8 hours, your hand and arms will thank you for getting a grip.
Camera steadiness, due to less right hand fatigue.
Optimum position of AF-On button. On the D7000, the camera is easier to use with the commonly preferred AF-On operation of the AE/AF-L button reprogrammed for AF-on. The button position is awkward in landscape orientation but is perfectly placed for AF control on the grip in vertical orientation.
Additional battery capacity, but that is not a major advantage with the current stingy D7000. With power hungry models like the D800 the additional battery in the grip is a big advantage however.
One thing frequently cites as a reason is better balance but that is not really a factor. Balance of a heavy lens is not related to camera mass or weight. A d40 is as well balanced with a 400 f/2.8 as a 55-200 light plastic lens. Balance is not compensated by camera weight, it is balanced solely by left hand support position. A front heavy lens means your left hand naturally moves out from the camera a bit. But a lens 1/2 the weight, the balance point means your left hand is moved in slightly towards the camera a little. We do that automatically when we pick up objects. The weight of a camera and lens is supported by the left hand and the right hand simple is used for maneuvering the read of the camera with a light touch, and operating the controls, unless shooting one handing point and shoot style, in which case you are not using a heavy long lens. With a grip however, shooting one handed with, say, a 70-200 2.8 is possible but not advised. Shooting one handed with a 85 1.4 is very doable with a grip however and just possible without.
Unless you are needing to keep the camera as small as possible there are few reasons NOT to use a grip.