I believe they mean some sort of transition from how the SmoothPlastic (basically default material) looks to a given other material, as seen here:
Especially for materials such as neon or concrete / sand, this can be useful for controlling the look of the material, and how rough it behaves - if I want to get whipped cream, I might want to use a sand texture to show the texture and feel of it, but not as appearant as the sand shown at 1 sharpness in the above pictures - because it should look smoother.
Edit: should be in #feature-requests:client-features , as it also applies to in-game use 