This effect is used to make thin diffuse objects translucent like paper or leaves. A diffuse layer usually reflects all the rays in different directions, so nothing goes down below to the medium of the object. Configuring the Opacity parameter of the diffuse layer helps some rays to go through the diffuse layer, so you can see what is behind the material. This is, actually, a sort of subsurface scattering on the layer-level.
Note that this works for thin objects only and the medium must be set to None, as described here.
Here is the scene of a light, green leaf and an orange plane. The plane is 30% opaque and you can see the shadow of the leaf on it, and even the leaf color:
Here is the same scene with 90% opacity and 100% roughness:
This time the shadow is still here, but the green color of the leaf is not that visible anymore.