@davewill: Because if a file changes, real backup software (like Windows 7 backup, and unlike robocopy) doesn't overwrite the original. It adds the changed file to the backup set, but also keeps the original.

So if a file gets corrupted, and you backup the corrupted version, you still have the old, uncorrupted version in there as well. Whereas with robocopy, if a file gets corrupted you overwrite the good version the next time you run the backup.