You're no doubt quite familiar with the use of color adjustment curves ('press curves' or 'device curves') on your devices. These curves allow you to control color reproduction, and set your devices to a known calibration state. These curves are indispensable, and there are tons of built-in and third party tools available to control these curves and 'dial in' your press state.
Adjustments made using press curves are 'global' curves, meaning they affect colors throughout the image. For instance, adjusting yellow at 50% changes the tone curve for yellow throughout the image.
Sometimes you can get away with this on a particular job, but of course your press is out of whack for the next job.
With selective color, you can apply an adjustment to the reds while maintaining the balance of the other secondaries, as well as the primaries, and the all important gray balance.