Nope, there ain't really nothing physically based here. Just a questionable design decision 😉
As already said ... if you set the default diffuse color to an off-white (0.8,0.8,0.8) it's like you set that because is convenient to have it as a default _arbitrary_ color for an initial lighting/shading setup. However if you set up the scale to be 0.8 is a different thing. It's no more a color choice, it's like saying.. arnold materials have a dimmed albedo by default .. and why on earth it should be like that ? More over, every damn time a user plugs a texture/shader/color in the diffuse color he has also to set the scale back to 1, that would not happen instead with a default diffuse color set to 0.8. And eventually nobody would ask why is setup like that 😉
