This appears to be covered in the official Unity documentation on Metallic Parameters.
By default, with no texture assigned, the Metallic and Smoothness
parameters are controlled by a slider each. This is enough for some
materials. However if your model’s surface has areas with a mixture of
surface types in the albedo texture, you can use a texture map to
control how the metallic and smoothness levels vary across the surface
of the material.
With a texture assigned to the Metallic parameter, both the Metallic
and Smoothness sliders will disappear. Instead, the Metallic levels
for the material are controlled by the values in the Red channel of
the texture, and the Smoothness levels for the material are controlled
by the Alpha channel of the texture. (This means the Green and Blue
channels are ignored). This means you have a single texture which can
define areas as being rough or smooth, and metallic or non-metallic ...
Based on that, if you've assigned a texture for the metallic parameter, you shouldn't need an additional texture for smoothness - one file functions as both the metallic texture map and the smoothness map simultaneously.
This was taken for the most recent version at the time it was posted (2018.2) for past versions, change the documentation version as shown here: