1. When the highlights are likely to be clipped or details lost in shadows as is likely when your subject is in the foreground of a bright sky in the background..
2. When the NR system of your camera produces unpleasant artifacts that you have no control over in the camera. Less likely in a DSLR but occasionally in my Fuji S6500, depending on the shot..
Shoot RAW any time you want the very best the camera can produce, or any time you might have difficulty with exposure or White Balance..
Nothing is enough for the man to whom nothing is enough...
There is so much more to this subject than why use it. First off what camera do you own? What is it's processing bit depth? What RAW image processing software do you want to use? In order to understand why you shoot in RAW you need to know all of this information to understand why working in RAW format is in your advantage. Many of the advantages increase the dynamic range of the image file created especially over a JPEG file. The answer to your question is not a simple answer, and you need to know the reasoning behind RAW. There is much more information stored in a RAW file. For example I shoot with a Canon 40D which use a 14 bit analog to digital converter.
If you take the same pixel in a JPEG file the information created is only 756 possible combinations. RAW format generally gives you increased image quality by recording the increased bit depth of the image which gives you a more complete tonal range information, more precise exposure information, and the ability to alter and or change almost any way that you want to process the image. It's a non destructive format that stays viable for many years...
When I first went digital, I always shot in Jpeg because I didn't care for the extra work. But after a little while, and after learning a little bit, I only shoot in Raw. It gives the most information from the sensor. I can always convert to Jpeg later..
It's best to have all the information and not need it than to need the information and not have it..
Scott W. McClure.
'You only get one sunrise and one sunset a day and you only get so many days on the planet. A good photographer does the math and doesn't waste either.'... The Late Galen Rowell..
As RAW gives you so many more levels (per pixel) in a format that can be easily manipulated far more than JPEGs, you have a much greater ability to recover from over and under exposed images, remove chromatic aberrations, change white balance, render greater detail with less noise from the image, process agressively, etc, etc..
Processing with more bits in Photoshop provides more accurate results with smoother gradients and less artifacts. For smaller output image size, differences may not be noticeable, but for larger outputs, eg large prints, the differences will be much more apparent..
Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..