What are ref.vtf files for?

I’ve been skinning, and every time I save I copy the contents of the original vtf onto the weapon_ref.vtf file, just darker. But all the ref VTF file is is a darker version of the regular VTF. What’s the point of it, and do I need to copy my changes onto it, or can I just leave it alone?

This is a link to an article that explains his better than i can (cant be bothered posting the whole article, so just go read it)

What link? :v: