Why a "digital" coax cable?

Sorry for the noob question, just trying to learn...

What is the difference between a 75ohm digital coax cable and a regular ol' RCA/RCA interconnect?

For instance, would there be a noticeable difference (improvement) between a digital interconnect and an Audioquest King Cobra for connecting the digital out from my Squeezebox (or CD, DVD, etc) to the digital coax input on my integrated amp? If so, how come?

Thanks all!