What a mainframe can do is have ANY single part fail, keep running at full speed and inform you of the part that failed so you can get it replaced. It is not unusual for any part to go all the way back to the substations!

Is this more or less useful than bunch of cheap PCs in a rack. Depends on what you want to do. The bunch of PCs model generally just accepts that there will be a few blips when (not if) something goes wrong and ensure nobody will notice. The mainframe model is even the smallest blip would be noticed so you can't have any.