Advantages of sound card

I really see no disadvantage to having a sound card besides the fact that it cost money. The advantages are better connectivity, better sound quality duh!!, and a little added performance in games.

I'm sure the purpose of the assignment is not so much you getting an answer from someone (I'm sure the person setting the assignment doesn't care how you answer), but more about you learning to do your own research. Asking someone to explain it to you so you can cut and paste doesn't really count, does it?

A high quality sound card is supposed to be producing a more fine tuned, crystal clear audio signal. It is said that if it is paired with a correct (usually expensive) speaker, you can actually hear the sound of a needle falling on the floor at low volume setting (SQ) or very low frequency bass you can't hear using standard setup (SPL). Not that I've heard of it before though.  

But there are more experienced people in this issue who can explain about sound cards better than I do, since I'm only using onboard sound card.