I have a question.
What does a photographer need a polarizing filter while using image post processing can fix everything? Especially if you shoot in RAW...
Megahertz and nanoseconds aside, a polarizer basically does the following:
1. Darkens your exposure by about 2/3 stops (depends on the brand and the type/amount of coating, though) to prevent blown-out highlights. An ND-grad is apparently better suited for this, however. This effect can be simulated in post-processing to a certain extent, using curves and highlight recovery, though the result may be too dull for comfort.
2. By definition, a 'polarizer' has the ability to shift the polarization of the light reflected from your subject (and anything nearby, at that) so much that it can rid the subject (or the surroundings) of any reflections whatsoever. For instance, it can completely eliminate reflections from the surface of a lake, allowing you to see (and shoot) something down below in absolute clarity. However, this really works if you have the light coming from your side, and on non-metallic surfaces only. This effect, AFAIK, can not be simulated in post-processing.