I assume it's more about when you don't have the level of microscopic detail required to model the texture of a material so you just average it out. E.g. brushe

I assume it's more about when you don't have the level of microscopic detail required to model the texture of a material so you just average it out. E.g. brushed steel is built from a material that has a metalness of 1, but in practice for most rendering you'll just represent the brushed microstructure using a metalness value somewhere between 0 and 1.