How does the Roughness Curve work?

When you go into a particle setting and check Children -> Roughness -> Use Roughness Curve, you get the curve widget instead of roughness parameters:

In fact, that’s pretty much all Blender’s manual says about this feature:

How does the Roughness Curve work? I’ve experimented a bit with the curve but am not seeing any obvious relationship between the curve and what I see on the rendered viewport.