What Dose A Sound Card Do

a few years back the onboard sound chips were cheap and did not sound as good as the after market creative labs ones. today some people doing high end audio work or with real good ears can tell the sound quality from onboard to aftermarket chipset.
some after market sound cards let you change the amp on the card to change the sound "warmth" to a users liking.
 


I Have An Intergrated One i was just aksing what would be the advantages of investing in one which u plug in to the pci
 


Thanks Any Way