I carry one of those when I go backpacking. The satellite transmission they send to summon rescue includes your GPS position, and they also serve as short-range beacons to help rescuers zero in on you when they get close. That can be crucial if the PLB can't get a GPS lock, because rescuers might only know that you're located somewhere along miles of road or trail. If you're unconscious and out of site a few yards from the trail, the beacon might be the only way for them to find you fast enough to help.