Here is a properly exposed portrait without the use of flash. It looks fine, but lacks any other interesting elements apart from the main subject. In actual fact, the sky and other highlights, are completely blown out because of the exposure I needed on the subject, so he wouldn't be under exposed due to the back lighting.
Exposure 1/60 sec. F4 100 ISO
Here, with the use of flash, distracting elements in the back ground are reduced or eliminated entirely, but the sky is much improved, and provides for an additional element of interest in the portrait. Flash will usually even out the total contrast of any scene, though I purposely created more contrast to achieve the effect I needed.
I under exposed my main subject by 4 stops, but popped flash to compensate for it. The result is, a more aggressive light fall-off, and a sky that isn't blown out. Now, you can clearly see detail in the evening sky but fewer distractions in the immediate background, thanks to aggressive light fall off. Though the subject himself looks like he received full and not fill flash, I feel the overall scene to be more interesting than the one that received no flash.
Exposure 1/250 sec. F8 100 ISO Flash