|My grandparents at work in a self-portrait|
Unchanged in over 60 years, are the principles of photography. These are based in the understanding of composition, lighting and luck (which comes from consistent practice).
As demonstrated in this photo, the photographer was focused on making the photo and the retoucher was focused on making the finished photo look its best. This is the core truth of good photography. There is the artist that makes the image and there is the artist that makes the image look it's best.
Either artist—the photographer or the retoucher—can be skilled at their trade (in fact, they should have more than a novice-level understanding of both trades). However, when it comes down to doing it properly, there needs to be a respectful appreciation for the differences between the two. These differences are more clearly defined as the two artists become more experienced. Eventually, you just don't have the time to do both jobs.
This dynamic does not apply to everyone. However, when you need a plumber—and you don't have full-time plumbing experience—you should hire one.