Um. This is not a "It's for future use!" wire. It's a blame SAFETY GROUND.
Take the obvious: There's a frame inside the Wall Connector that's tied to safety ground. Now, suppose a piece of nifty insulation, on a wire, in the case, or some piece of electronics, decides that Now Would Be A Good Time To Short between one of the hots to that frame.
The world's biggest slug of current Every Seen is going to flow through that short to the ground. For the amount of time it's going to take, it's not going to be long enough to get that 10 Ga wire to melt, but it will be long enough to pop the breaker. Not to mention any GFI hardware present, which I understand both the Wall Connector and Mobile Connector have built in.
Without that safety ground, whatever passes for a ground frame inside the Wall Connector (or the car, for that matter, it gets that ground, too) is going to end up at one Hot Voltage Potential or The Other. And, if somebody touches that it's-supposed-to-be-ground-but-it-ain't, that person ends up dead as the current flows from that high-potential ground, through the human body, and to that damp shoe in the puddle in the ground outside the house, or wherever.
Double-insulated power tools don't need the ground because it'll take Two, count 'em, Two sets of insulation to break down before Death.
Back in the 50's before NEMA5-15 (with the ground pin) sockets became common there were deaths all over due to fun like this. Imagine a clothes washer with no ground wire attached and all those motors and water on the inside.
If I'm not mistaken, I think the Wall Connector, Mobile Connector, and possibly the Tesla itself does a check to make sure that the safety ground is connected before every charge, Just To Be Sure. (Which drives external GFI hardware nuts, if said hardware is sensitive enough to notice the check.)
