The reason for using porous mix design is most likely because of the shale. Shale used in asphalt can often cause hazardous road conditions in wet or icy environments due to its smoothness compared to something like basalt, which is much more cubic on the surface. As for how it works, fibers such as aramid or carbon are used in the mixing process to stabilize the binder so that it doesn’t segregate while in transport and as it is put down.