As far as i can tell subd and smooth is really 2 operations, one that subdivides all the geometry and then one that smooths it. The plugin lets you do both of these at the same time or just one at a time. So if youve got some geometry that youve already subdividedbut hasnt been smoothed you can smooth it without having to subdivide further, which would produce no-quite-right results and very high poly geometry.
One more slight difference is that when it (smooth) has finished changing the geometry it doesnt soften all the lines.