As an example, the picture below was taken before sunset with the sun setting behind. This picture could not be achieved with natural light alone as the subjects would have been dark silhouettes. The flash lighting that I used highlighted their faces, while maintaining that beautiful golden background glow.
Fill flash can also be used when there are harsh lighting conditions to fill in shadows (under eyes etc) which is much more flattering. Added light also helps to make images sharper and can help reduce graininess.
Natural light photographers are very limited in when and where they can take photos and the wrong lighting conditions can really make for a bad photo.
While I always try and shoot at the right time of day (the golden hour), that is not always possible. Most of the time I use some sort of added flash (even if its subtle) and find this produces the best results.