The Mix tool is what it has that you don't find in Snapseed and elsewhere. It lets you adjust the luminosity of different colors in the conversion to grayscale. (I think luminosity is the right word.) It's similar to the B/W filter in Photoshop CS, if that helps?
If you just desaturated a color image, you might find that a green and a blue came out as the same gray. With the Mix tool, you could make one a lighter gray (and both darker than reds, etc.).
If you don't need that particular trick, then I don't see any big advantage for Camera Noir.