LOL Frag... you beat me too it. I was actually going to post the flash info right now. All the photos taken with flash will have a flat feel to them. Without using the flash you will have more of a three dimensional feel providing a more appeasing feel to the picture. Also the color will be shifted like Frag has mentioned. Most flashes will be of a 5500K temperature on the digital cameras. Which is going to provide with a very yellow look to the picture and drown out the colors that we want to capture and show off.
Most of our lighting is at a minimum of 10000K temperature range, yet this is more of a whitish-blue hue as you go higher up the Kelvin range then the color becomes bluer. We want to catch these colors as it is more appealing to our vision. Hence the use of actinics with metal halides and/or daylight bulbs.