JPG files are 8bit, this means the number of different shades of any particular colour is limited. We take the RAW photograph and render each image from -2EV to +2EV so the best possible range is available for every image.
* EV stands for Exposure Value
When should I use a particular EV?
In general, you should choose the image exposure that best matches the artwork/shot you are working on. E.g. a night time shot would require a darker (-2EV) image.