somehow it had never occurred to me before - most practical PBR guides describe "roughness" in a rather handwavy way, so you get used to thinking about it as just some number, but it actually has a very precise meaning.
trowbridge-reitz/ggx models the distribution of micro-normals within a surface patch as if the patch was part of a perfectly reflective ellipsoid.
alpha controls how squished the ellipsoid is: zero alpha = no curvature at all (degenerates into flat disc) so all micro-normals are oriented the same.
in the anisotropic case, as far as i understand, the two alphas are slightly different: they control how much the ellipsoid is stretched in the tangent/bitangent directions.
relevant bit that defines the parameter of trowbridge-reitz distribution as the "squashedness" of the ellipsoid ("e" is basically alpha here, replace sin^2 with 1-cos^2 and the usual form will become apparent)
