To help prevent ground loops, which occur when there is more than one ground path. Often misunderstood, the ground conductor prevents the chassis from becoming live if the AC line touches it. We use double insulation instead, something that may sound unfamiliar, but you've seen it before on power tools. This method may cost a little more, but no one can resort to using a 3-prong to 2-prong adapter to open a ground loop ("cheater" plug in this case since the green wire is used for decoration instead of safety).