ok there are basically 3 reasons y u might want to buy it.
1) When u take a vertical photo, do u find ur right hand in the upward air position, and is very easily prone to handshake? the right hand wrist got to twist and support the whole dslr, its very tough on the wrist and hence more handshake..
Vertical grips(dun count 3rd party ones) got shutter release, so that when u tilt it to shoot in vertical position, ur right hand can grab the battery grip and press the shutter release on it, just like taking a normal horizontal photo..
2) Its called battery grip becos normally it can take 2 batteries instead of 1, so u can take more shots.. some battery grips also allows u to use AA batteries to shoot
3) To boost ur ego, battery grip make ur overall camera looks bigger! and maybe look more pro to the normal people...[becos bigger = more pro ma ]
Battery grips in SLR/DSLRs not only enable u to load batteries in it for extended and flexible battery types, it also allows you to take portrait oriented shots in the usual way as you take at landscape orientation without having to tilt your right arm up which do not provide the support. That's why they have a shutter button with the dial on it to take your picture and change apertures.
Behind also have the same exact buttons as you have on the top right hand side behind the camera which includes AE lock, etc...
Older battery grips like the MB-D100 included a voice recording option as well on the battery grip and including the ability to use 2 batteries simultaneously at the same time and allowed switching of any one dried cells with a new one without affecting camera operation.
If you don't have the cash, get the camera first, the grip is pretty much secondary to some.
Personally, I like the feel/build it gives to the entire camera as a whole and with my constant shooting of portrait orientation, it's pretty good to have one.
The shutter count number can be reset but the actual count on the shutter cannot. The number is just as a reference for u to keep track as every camera's shutter count is limited to a certain cycle. Once the shutter count is up, then it's time for u to either send to the service center to change the shutter, or buy a new cam.