It's in case the plug gets loose and leans down, the ground pin will be what gets contacted, not the hot & neutral.
If the cover happens to get that loose, and the plug is hangin half way out of the receptacle, the cover is more than likely going to swivel and short out from the side long before it completely falls off and drops down across the hot and the neutral. if it gets this bad before somebody decides to do something about it, they probably deserve it shorting out on them.
JAP.