Retouching can allow correcting small skin imperfection that is unfortunately here on your shooting day. 

You prefer not to have it on the portrait you offered yourself ; retouching can greatly help in that case.

For instance, here are a few before / after. A spot close to the eye which will be removed by the doctor later on. A tooth which recently dislocated and which will be corrected by the dentist in the future. A few unwanted spots. These things will no longer be visible on the person and are unwanted on the photo portrait you'll keep for years. Your photographer can help in this case upon request.

I filmed my screen to show the difference between a calibrated screen vs non calibrated. The color on the attached picture is not ideal since this is filmed, it's very low quality, however it demonstrates the difference between the 2 screens.
The result if your screen is not calibrated is that when you'll print your photos, you'll likely get prints that will look darker and more contrasted (similar to the calibrated version) than what your screen displays (non calibrated).
One can calibrate a screen by using free softwares that can be found online however calibration in this case is done manually and is not accurate. Much better, you can buy or rent a calibration device which will automatically adjust your screen luminosity and colour accurately through a scan.
This way your screen will accurately reflect colours and luminosity and your prints will be closer to what you visualised on screen and had in your mind.

All my photographs are taken in Raw format. Why : because I do not want my DSLR to decide the final result of my image. Raw format allows me to refine my image exactly the way I want it to be. Whereas with a JPEG, my DSLR would automatically apply global sharpening and saturation to it. 

The other big advantage of Raw format is that it contains a lot of color and light information, it gives more details in light, shadow, and color. Post processing is thus more precise and flexible. A Raw file can easily be worked on, without impacting quality. Which is not the case of a JPEG which contains less information.

Why do I post process my photos? A Raw format is not a final file. Sharpening, colors, contrast, light and shadows need a little bit more work. It is "raw" and flat. It needs action from the photographer, such as a film would need the professional service of a lab for post processing and to get to the final image.

Post processing also allows the photographer to finalize the style and atmosphere of a photo, bring his/her own personal touch, and convey the emotion of a scene.

This is what makes the difference and therefore it is a crucial stage. It can take a bit of time, depending on the photography made, but it is worth the wait.

Once post processed, the image is saved as a JPEG for sharing and reading through other computers.

Author : Rev'elle Photo, Lyon based photographer (France), http://www.revellephoto.fr