The Roughness Channel outputs an image containing the Roughness value of the materials in the scene. High Roughness materials will appear white in the image, while low Roughness materials will appear dark.
This information is useful in compositing, when you need to separate rough areas and reflective areas, to perform certain adjustments (i.e. tweak reflections, tweak the color of diffuse surfaces, etc).
Render (left) and the Roughness Channel (right), showing the specular areas as dark (lower Roughness values) and the diffuse areas as white (higher Roughness values)