Quality Scanning reads the disks & produces charts of the quality of the burns.
You'll see loads of images posted of such scans & these can be very useful in seeing how well the burner/media & burn speed combination has worked.
Problem is that once you get into this it can become a bit obsessive. A bit like my son & World of Warcraft.
Personally I do this with every DVD I burn & sometimes because of it I'll re-burn one. But a good friend of mine uses crap media & burns away & then wonders why in weeks to come that he can't play the discs on his standalone DVD player.