Here's a quick and dirty example of how you can use more than the centre AF point, even if you only use one AF point at once, being able to move it around helps a lot, especially in portrait situations.
Here's a shot I took at my missus' latest book launch, not the best subject but it illustrates my point nicely. Putting the camera in portrait orientation, the centre AF point would have focussed on his chest, moving the single AF-point in use to the far end made his head in focus instead (which is what you want, a sharp chest and blurry head doesn't make for a good portrait).
Screenshot from DPP using View->Show AF Point (Alt+L)
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