I've just done some experiments in order to better understand why the "Normal Texture Coordinates" work so well with the creation of eyball veinlets. I haven't found a satisfying explanation so far but what I can say is that "Normal Texture Coordinates" obviously introduce more variation by reflecting the mesh's topology as is can be demonstrated with the "Triangle Fan" of a default "UV Sphere":
1) Object Texture Coordinates
2) Normal Texture Coordinates
"Normal Texture Coordinates" furthermore introduce a significant contrast between darker and brighter areas as a kind of filter on top of the "Texture" itself (Note: This is only a visual but no technical description!):
