Digital Light & Color

What would be an example of using Colour/Primaries transformation (i.e. when is it more useful than other colour corrections)?

There are a few examples in Transformations.pdf.

Primaries corresponds roughly to the Calibration settings in Lightroom which were originally intended not as a color adjustment tool but rather as a way to adjust to different RGB primaries of camera sensors. Some people claim that adjusting saturation by playing with these settings produces better results than adjusting Saturation or Vibrance in Lightroom. I have not used it extensively or done tests myself yet so I can't say if this is better or not in PWP.