Hi Lee, Suppose you had a skyline or horizon with buildings with windows,frames etc. You may want to sharpen the sky edge and windows, but not the textures of the buildings. this is one reason for the option.
Try this for sharpening, instead of the USM.
Duplicate the layer needing sharpened.
Run the Filter > High Pass > experiment with radius settings.
You will see the edges, almost like the Emboss filter, against a mid-grey.
Set the Blending to Hard Light and adjust the Opacity.
Experiment with the other Blending 'Light' modes.
I use the Colour Range selection as my weapon of choice for masking.
As you say it gives greater control than the Magic Wand ever could.
Keep exploring, there's tons more for you to discover.
Cheers Al[/b]