Let’s say you have a cube with white Emission material, rendered in empty space. The ‘Film’ render setting is set to transparent, so if you rendered the scene to output as an RGBA, you would get a transparent area around the cube.
Now let’s say you switch the output to RGB or BW. The empty/transparent space would be filled with black, but what if you wanted the empty space to be white on a BW or RGB output image? You may say that it cannot be white because the cube is in a dark empty space, therefore the antialiasing would need to put black bits in the antialiased edges. It might also be said to make the World Background white, but doing that casts light onto the cube and you might not want World light to effect the cube.
Right now, I am having to render it as RGBA then use Compositing Node Editor to fill the transparent hole with white.