Cameras don't see the world quite the same way as our eyes.
For better or worse, our eyes (and brain) are very forgiving. We don't really notice the wrinkles, age spots, stray hairs, scars, acne, blotchy skin colour or bloodshot eyes on friends around us. But a camera can record every flaw and blemish in exquisite detail.
We also don't notice things like telephone poles or power lines in the way, flickering computer or TV screens, uneven lighting, odd reflections, weird shadows or converging verticals.
A camera captures one moment forever, unlike our ever-changing and always-moving eyes. When we look at a photographic moment, we can see all the things that our eyes missed.
Standard post-processing tries to make a picture meet our average expectations. It also includes making adjustments so that it will be easier for the client to use the finished photos. Retouching helps a picture exceed our expectations.
Here are a few Before-and-After pictures. The Before is the original image with no post-processing except perhaps for some cropping.
Roll your mouse over each photo to see the Before version.