A good question. For many years I was a painter. Like many painters I developed a set of techniques for checking the progress of my painting. Things like:
Standing far away from it, looking at it upside down, looking at it in a mirror, looking at it in a dark room (good for checking tones).
All of these methods can be easily replicated (and bettered) in PS. But I have always wanted a way to separate saturation from tone and color (Which I know would not be possible in the real world). I can do this in Shake quite easily (people who use Shake have a wonderful set of color correction tools) but have always been frustrated by PS inability to do it (tho oddly enough in in PS 1 it was possible).
avinashlobo: you are mistaken. I do not want to saturate/desaturate. I want to know which bits are saturated via a grayscale map.
sorry the late reply. bin busy.