Misdirected lighting causes poor vision and dangerous glare. Safety is reduced and driving hazard increased. Misdirected light is useless and wasteful. A fixture scattering light into the neighborhood and sky needs a higher wattage bulb than one that only lights your property. A fixture that only lights your property uses less electricity.
Unnecessary light causes unnecessary air pollution.
Excess light disrupts circadian rhythm of flora and fauna. Recent studies suggest lighting can create health risks.