Why does a compost pile need turning?

Composting involves the reduction of organic material to a more concentrated form of matter. It is esentially a (very) slow cooking process. The heat required to do the "cooking', requires oxygen, just like any other heating process. So the benefit of turning the compost pile is you are adding oxygen by creating separation in the layers of compost which will compact as the pile ages. A secondary benefit is you redistribute the pile so that all of the organic matter gets to be near the center at one time or the other. The center is where the heat is the most intense. By regular turning of the pile you will have better compost in the shortest possible time. However, do not turn the pile too often as that will prematurely disturb the heating/composting cycle.