Group Re-lamping is best practice for high intensity discharge lamps commonly found in exterior lighting applications to maintain optimum light levels and
light distribution. 
Exterior light levels are at its peak when HID lamps are new and reflective lumenairs surfaces are clean. Many HID lamps depreciate quickly resulting in loss of light and color-shifts. This is often found when lamps arapproching end-of-rated-life which can shortens ballast operation adding more costs to replace ballast & capasitors.