The Pentax grip really 'makes' the K-7 for me - suddenly its a proper serious bit of kit. Key advantages (apart from those already stated well by all above me - controls, weather sealing etc) is the flexability of battery options - either an extra camera battery, or 6 AAs, very handy for shooting in remote locations where there's no electricity to plug into and you should be able to find a shop selling these batteries very widely around the world.
My grip is a permanent fixture - it and the camera are one, and the handling is improved a lot - very easy to jump around into vertical shooting and back. Well worth the money!