Here it's usually because a road gets developed and the city requests more light. To save money though, every other light on main roads was taken down.
when we get new streetlights.....its usually on existing poles...though with recent road developements here metal poles have been erected to give light to previously unlit stretches of road.
sometimes installations like this won't be compleatly new....sometimes they ditch the Long Guy wire arms and install a shorter arm....like the one above...i've even seen streetlights moved for no visable reason.