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:
https://docs.blender.org/manual/en/latest/physics/particles/emitter/children.html#roughness
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.