Originally Posted by AlphaDog11790
So that you dont zap your components with static electricity.
It's the user which is the problem not motherboard or any component.
Originally Posted by Syrillian
HDDs and Mobos can be grounded via the standard mounting holes and the screws that secure the hardware to the chassis of the case (standoff / HDD rack).
HDDs would be actually harder to destroy by ESD thanks to their metal casing unless you touch PCB directly... and in that case HDD being grounded doesn't help because high voltage would discharge through low voltage components.
That's why you should always touch metal structure of case before jamming fingers into components.
By the book way would be keeping also case on grounded antistatic surface, but I've myself used only wrist strap and touch case most of the time with other hand to prevent buildup of potential difference between case and myself or always touch case first myself instead of touching case/components already in it with component being installed.