Roughness is only included in the API as a default value if, say, a plugin, would access the data and create a material in an application. Then it would get an approximate roughness value, so glass is glossy, and concrete is rough.

There's however too much variability in roughness values since it's a property of an item rather than a property of the material itself, so a drinking glass could be very rough if worn, or very glossy if new. That's why I haven't added it as a property on the website.

Hope that makes sense!