It also gives your pinky finger on your right hand somewhere to rest as well. I don't have big hands at all, but even I found that without a grip my little finger would be waving around in the air - or rubbing up against the corner of the camera, which is annoying. A grip gives you that extra little space.
Battery life is also better in a grip - running of two batteries at once means that you can shoot for ages without having to worry about changing batteries -- but it does mean that you want to carry 2 spare batteries for when it does die.
The vertical controls are also great to have - even if youre not doing much/any people portrait work there is plenty of scope out there for the portrait aspect.