Your footage was shot on different times of the day, with different color temperatures, using different lenses and bodies combinations, you need color correction to make your project look uniform. There´s some color in the background you don't like, some over saturated object in your shot distracts the viewer, you need to blur somebody face or a logo, you need color correction. So you want your film/video to look a little more cinematic, using the correct colors to support your narrative, and enhance your creative vision, you need some color grading.