If you look at a diagram of the lungs, you'll see that the bronchioles branch in many different directions. In an excellent airway clearance video, Noah Greenspan explained why changing your position and angle is so important in bronchiectasis, as it helps target different airways. He compared it to dipping an apple in caramel. You have to rotate the apple to coat the entire surface.

If your bronchiectasis is in the lower parts of the lungs, you may need to rotate your body, or even position yourself so those airways are nearly upside down, to let gravity help drain mucus, much like turning a ketchup bottle to get the last bit out.