I am sure someone else can answer this better than I but I will give my opinion. As an animal trainer and if you have worked with birds, you should know that most birds (daytime hunters) become dormant in the dark. They relax, calm down, pull up a foot and mostly sleep in the dark. The hood serves the purpose of putting a bird in the dark, per say. This reduces stress, keeps the bird calm and also allows the falconer to do things needed while the bird is hooded (coping, equipment changes or adjustments, medical purposes, etc.). Probably the biggest advantage of hooding a bird is during transportation to the field. These are a few of the things that come to mind for me. As I said, I am sure others have a better explanation.