The ground and neutral lines are spliced together at the fuse box (and generally again out by the local transformer). So when you’re wiring up a house it’s important to keep track of which line is the 120v line and which is the neutral line. So that’s one use for the nearly useless ground line. So the reason that ground lines run to outlets is that every now and then it’s nice to have access to a 0 volt, 0 current wire. But it’s not really that important, which is why so many outlets don’t have a third hole.