To get sunlight. Look at a woods sometime along the edge. Trees do not always grow up. Sometimes they grow up a couple feet and then turn sharply toward the light. I cut a 60 foot cottonwood tree in my woods about a month ago that was growing out over our yard about 6 feet off the ground and the stump was 42 feet back in the woods. I was very careful with that one. All that weight hanging at about a 30 degree angle makes them prone to snapping off or barber chairing when you start cutting. Good way to get killed.