Why do digital coax cables matter?

Could somebody please explain this to me? Why does a $100 dollar cable sound better than a $50 cable. Why is silver coax better than copper coax? Why do the quality of connectors matter in the digital realm?

I'm currently needing a cable for a Stello U3. Some people claim that are no discernible differences out there b/t different levels of coax cables. They say the only things that matter are impedance,cable length, and adequate shielding.