Sound Cards & Video Capture
Sound cards provide higher quality sound by delivering more power to headphones (particularly useful when using high impedance headphones in the 25+ ohms range).
When headphones are low impedance (most normal headphones), a sound card will provide a "fuller" sound. The listening experience is noticeably better, depending on the headphones and the individual.
Do I really need a sound card?
For everyday users, most likely, no.
Sound cards are for enthusiast gamers or audiophiles. They make highs crisper and lows smoother. When paired with the right headphones, it's easier to distinguish sounds, which includes directionality (this can help you hear footsteps in-game).
Sound cards help you immerse you in the sounds of games and movies.