My first reason for a grip is for the extra buttons/dials in portrait mode.
Making the camera bigger is nice for my big hands.
The extra battery power is a bonus but handy for the higher frame rate if your camera supports it.
Weight can be saved by not putting a battery in the grip if you don't need it.
My first reason for a grip is for the extra buttons/dials in portrait mode.

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Joseph K
Seattle, WA, USA

D700, D200, D70S, 24-70mm f/2.8, VR 70-200mm f/2.8 II,
50mm f/1.4 D, 70-300mm f/4.5-5.6 VR, 18-70mm f/3.5-4.5 DX