Update Changed Files
Make all files in the backup directory be up to date with those on the source device.  Do not remove files from the backup when they no longer exist in the source folder.  This might be useful you want to archive data so that it can be removed from the hard drive.

Duplicate Exactly
Make the backup folder an exact duplicate of the source directory.  Remove any files in the backup that no longer exist in the source directory.    This would be useful in keeping a mailbox up to date where some messages with attachments have been deleted.