Glossy shader alone doesn't indicate metal - it indicates reflection. So it is very common in shader node groups because every material is reflective one way or another (except of some vantablack, emissive materials, etc.)
However the shader you presented is not physically correct - thus it is an example how not to construct shaders.
You don't want to mix in glossy node through mix-node with constant mixing factor - you need to mix in glossy through fresnel:
How do I create a reflective white material?
When you have a rough surface you need a specially constructed Fresnel node that accounts for the roughness (image borrowed from blenderguru but original credit goes to cynicatpro):
So if you add a glossy node correctly to any material (shader), you are just adding another layer of reflection - basically you are adding a clear coat - you are layering.
On some materials this makes little to no sense - like on glass. It makes a look of heavier glass - this can be achieved with IOR better.
If you need a metal shader, you are good to go with one glossy shader. Metals don't have any diffuse component:
This is as complicated as you will probably ever need. It has a metal rim color control and metal IOR control which you can unplug and remove for simplicity.