Why is it important to use vice jaw plates?

The jaws of a vice used in metal working have small teeth like segments cut into them to assit in gripping metal. If you use the vice for soft metals such as copper or brass the jaws will leave indentation the in the metal which will prove very difficult to polish out. plain jaws or plates are added to stop this. The same would occur if you use these types of vices to grip wood or plastic.