In the real world, metals interact with light in a very different manner than other materials. Internally, they have a diffuse color that is completely black, and have a specular color that is very high. Non-metals on the other hand primarily have a diffuse color, with a fairly low, and monochrome specular color.
Instead of having the user swap black into the relevant texture channel based on a material being metal/non-metal, Alloy shaders user the 'Metallic' input. This input feeds an interpolated value of your Base Color, and black into the appropriate parts of the PBR equations, allowing you to easily make a material behave as a metal, or non-metal object. Values between '0' and '1' of metallic are incredibly uncommon for real-world materials, and should generally only appear on the map at transitional or noisy zones for weathered or composite materials.