Many people prefer to not use the shutter release to focus, and I am one of those people. I want my camera to focus when I want it to focus and I don't want it to re-focus when I press the shutter.
For an example, I find this useful when shooting birds in flight. I can keep my finger half-pressed on the shutter, giving me VR if I have it on, but stopping the lens constantly trying to refocus if I lose the bird in the frame. When I get the bird in the frame and under a focus point, I use the AF-ON button to focus, keeping it pressed to track, then shoot. Using the AF-ON button also allows for adjustments in composition after obtaining focus, by releasing the button and then recomposing. Often I find this easier than constantly moving the focus point around the frame. It also allows me to use the centre focusing point for faster focusing speed if conditions are not ideal. (I normally use 9 or 21 points in good conditions.)