IMO having an independent source to handle audio makes your computer work not as hard and frees up some of it's resources.
I'd say everyone should have a sound card where applicable.. Some games don't play well with onboard sound at all. They just can't handle too many sounds at once, not to mention they can't handle top tier quality as well. A dedicated sound card however, will handle them all and ask for more..
Example, with onboard sound, some games you play, you might actually have to edit the config, to limit the amount of sounds a game can make at once.. A sound card helps alleviate issues like that.. Some games will totally lag out and crash because of this problem. It is very well documented.. GL and whatever you do.. Enjoy.