Shadow Copies lets users recover previous versions of files they're working on. This can help reduce support calls like "Help! I need you to restore file X from backup!" So it's generally a good idea to enable Shadow Copies on Windows Server 2003 file servers when your desktop computers are running Windows XP.
Shadow Copies comes with a price though--every file that's saved is written twice. Most file servers can handle this, but if your server is under continuous heavy load then enabling Shadow Copies can impact performance. The solution is to configure Shadow Copies storage to use a separate physical disk (spindle) than the disk hosting your shared folders. To do this, open the properties sheet for the volume where your shared folders reside, select the Shadow Copies tab, click the Settings button, and specify a different volume (separate spindle) for shadow storage.