This helps considerably when you have attached long and heavy zooms & primes to your camera since this acts as your counterweight. The shutter button on the grip is also very handy when you are shooting in portrait orientation. The joystick and other controls mirror those on the camera body,too.
I hated battery grips before. I felt that they were unnecessary since they did not add too much functionality to the camera. It wasn't like how it was before when battery grips actually make your cameras into something else like making it shoot faster, rewinding your film and other things. All they do now is serve as an additional source of power and adds a little bit of speed to your FPS when you attach a pro body battery with it. All of which are not as essential as what I previously mentioned with how they used to work in manual film cameras. The most that they offer now is ergonomics.
I permanently damaged my right wrist many years ago and I shot an event from morning till night with a D90. Almost all of my photos were shot in portrait and it strained my wrist. After the event, the first thing that I did was to buy a battery grip ASAP. The grip prevented me from hurting my right wrist further since I do not have to twist it when I shoot in portrait. Not only that, I was shooting with a heavy Tokina lens back in those days and so the grip acted as a counter weight. It made the handling of the setup so much better since everything is now balanced.
Having the removable grip also gives me the option to go light or go heavy. I most certainly will not use the grip with my camera when I would should landscapes but when I need to shoot events or the ramp, I have the option to attach it and save my myself from unnecessary pain and discomfort during the shoot and the day after that. Rick.