I prefer onboard...some people think externals are better. It can be subjective...like that videophile that swears he sees an extra pixel when he's looking at the screen from 2 inches away. It can apply to computer audio people too...they'll swear it's that much better. To me it's just sound, lol.
I haven't purchased a soundcard since the days of soundblaster 16 and when Creative made you buy your drivers via snail mail.
I did actually buy a sound card....when I built a comp that didn't have it in the mobo. But not creative, since I wanted real quality
All in all it can improve your system performance about 1%, offloading some of the processing...etc