It's a surface area maximization strategy. The hole maximizes the surface area, which is important for cooking when the doughnut is fried in oil or baked. The closer to the heat source (the oil) the innermost part of the doughnut is, the faster it will cook, and the less time the outside will overcook while the inside is still under-cooked. So basically the hole is for better cooking, so that the doughnut will taste better and will cook evenly.