Guys, the WB adjustment itself does have an effect on the shadows, but it has more effect on upper shadows through the highlights. As has been stated, the deeper shadows are often affected by the color reflected into those shadows, and they can have a significant color cast all their own. By balancing the shadow levels to the same physical point on the curve (not the same numerical value), you can partially or completely offset a color cast while pre-biasing the channels to assist in contrast adjustments. Sometimes this is all the contrast adjustment that needs to be made. In other cases, you either want to pre-bias the shadows using the LCH Lightness channel to move the black point up in all channels together, then bias the individual channels, or do a global RGB composite channel adjustment after the individual channels have been adjusted, or both.
Clear as mud, right?
Simple answer? It works. It's fast, easy and repeatable.
Nike. (sideways reference to their slogan)
Ron