Soundcards offer several advantages, ranging from better signal quality, to a (significantly) better Dynamic Range, not to mention other audio enhancments that onboard audio does not offer [Virtual Headphone/Speaker, etc].
Anyone who says soundcards don't make a difference is deaf. To me, not using a soundcard is like playing a game with 0x AA. Its an enhancement, not a necessity. [Yet some people here would glady pay upwards of $500 to move AA up one more notch...]