It's all about the preamps and the A/D to D/A conversion quality. A top end sound card will produce the most accurate playback in terms of audio fidelity (Digital to Analog conversion, or D/A). Likewise, a top end sound card will be able to record an instrument (guitar, bass, vocal, turntable) with better results (Analog to Digital conversion, or D/A).
Of course, the latency issue is key as well in that an internal sound card will always have increased latency as opposed to an independent sound card/audio interface.