This allows any image to be used as the background image for a website, as a module, or even for advertisement.
When the browser is re-sized the background automatically re-sizes with it. It keeps the image a proportional size to the window size, and it always keeps the image centered.
It prevents images from becoming pixelated due to increased size. It keeps images clean, and distortion-free so that any image you use looks great. Also, it automatically dims the image to leave user focus on content, and not the background.