The intregity of the drive based on how well the low-level
format is done.
When done at the Manufacturer, the enviroment conditions are optimal.
When done on a PC, a simple hiccup in the format will render the drive either
useless or do more damage then there was prior to doing the format.
(a lot of factors that are not an issue when the Manufacturer (ie. voltage drops, motherboard
stability, Bios etc..) does the format)