No matter how hard any photographer tries, some images aren't perfect. Stuff happens.
"Problem" images, or images that are under-exposed, have incorrect white balance, have picked up an odd hue due to light sources or reflections, can usually be corrected, at least to a degree.... Normally we don't need to "fix" images - we're good at what we do, and we always take a number of shots of the same pose or moment, just in case. But since we're often asked about digital editing, here's an example of a time where stuff did happen.
In the image above, just as the shutter tripped a lady with a red umbrella blocked the strobe. (The other four photos in the series were perfect, but this one clearly wasn't.) Not only was it under-exposed and muddy, but it also had a pretty bad orange/red cast.
It was definitely fixable, though. Here's the same image, in black and white, after about 30 seconds of digital corrections including adjusting levels, curves, and some selective dodge tool application.
Not bad, right? Here's the same image in color, after corrections. This version took almost five minutes to correct; the main difference in the black and white and color versions is the need to apply adjustments to ensure skin tones look natural (which can be a lot trickier than it sounds.)
Obviously we don't want - or need - to do this kind of work to every photo, but sometimes we capture a moment or expression we'll do just about anything to save.