Material Matte Layer is a tool to make much more easy compositing session, think about it like a object matte in c4d compositing tag, but at material level, for example if you have a scene with some back object an one character,with black hole mode you can render a static backgrounds character with in black Hole mode and then you can make another render with character and back in black hole, with this situation if you need to change some thing in character, you can rerender only it and not the back, you can use it to isolate object and fake inclusive or exclusive lighting in various part of the sceneand it can be used to make object and material ID with AA and Physical Camera.