It makes sense to me but it helps to have some background as to why. (I'm not an expert so don't put too much weight on anything I say but I generally know the gist of things) The metallic value is for saying what type of material it is and what shader model it's using. I'm simplifying but a non-metalic material will have…
*edit* nvm, I get what you are asking now. but for non-metals it give more control over the specular independently of the diffuse value for example, you cant do this in Marmoset I think while keeping the same diffuse.