Because in digital compositing, 0 means completely transparent, any value above 0 means semi-transparent and 1 means 100% opaque, thus if the value is 0, the rendered caustics on dummy CG surfaces can be composited with live-action footage seamlessly and without showing any colors from the CG dummy surface on the footage. However, there is a known issue with Maya the mental ray engine, which is not fully compatible with Maya nodes.