Specular layers are all about reflections. If you need a material to reflect something, you need to enable the specular block in its layers. There are two common usage cases for specular layers:
Use the links above to get more details about each usage case, also read about thin-film interference effect that you can do with specular layers.
Koru has a large materials library with lots of different plastics and metals that you can load into scene and have a closer look at how they are done. See the rough materials (done with Roughness parameters), see brushed materials (done with Anisotropy parameter), see various glasses to get a better idea of how the materials work in Koru.