A good explanation is from Starizona. Way at the bottom, there is a picture to illustrate why this is better.
http://starizona.com/acb/ccd/advtheoryfoc.aspx
"...When an off-axis area of the CCD is used to focus, more of the curved focal plane falls within the depth of focus and a larger area of the CCD will be in focus...."
SGP Autofocus works great on my SCT, but I would like to minimize field curvature effects.