Color grading separates amateur work from professional grade. Through proper color grading you can set the mood, time period, aesthetics, and ambience for the piece. Color grading is essential in correcting exposure as to achieve the Hollywood reflection level. At times mistakes in exposure while shooting will destroy image information for ever, but the grading process can artificially bring back tones and shades that where lost as shot.
Image matching is critical, sometimes you may have the task of matching images that were shot at different times of the day giving a different color temperature or exposure level and this will require that you go in and correct them so they fit visually next to each other.