this help you to continue work without worry for the change size in differents devices or that your UI deformed the objectsuse the aspect ratio for save the dimentions of yours UI objects, like, images, buttons, Inputfield, etc.In addition, too its have the black bars that help you that don´t look more than the necesary, or if you works with UI with aspect ratio fixed and camera in full screen you could cover the background for you interface look like you want
