Allow to create transparent buttons
This can be useful when the layout background is professionally designed and you want just to mark where the keys are placed.
This is great for gamers. Would allow me to use the developer provided key binds image and just use transparent buttons as hotspots. Would also allow me to use PhotShop/GIMP app to make my own backgrounds as nice as I want and then just make the image the background and use transparent buttons for the key commands.