Am really considering getting an external flash unit for my Canon setup (580EXM2) and am quite new to the whole area of flash photography. I have gotten away with using the pop-up flash on my camera when doing portrait shots, I underexpose the flash by -1 in order to not get that "in your face" flash kind of look. I've attained some good results but probably no where near as good as using a dedicated flash unit. So to all those experienced external flash users out there, can you dot point some of the key benifits of using an external flash over an inbuult one when being used for portraits, and/or creative uses.

Cheers
benefits