Cuts the odds of your buying the farm if you make a dumb mistake. There was that guy at a nursing home who got killed when trying to remove a broken light bulb from a light fixture he thought was dead (the drawings said that all the lights in the room were off the same light switch, but it turned out the building wiring was modified to make that particular light stay on all the time, off an emergency lighting system, and the drawings not updated). Sure, he should have tested that specific fixture, but...