I have no idea why Bethesda have included a couple of settings (like God Rays on Medium/High/Ultra) that absolutely make no visual difference, but are very taxing. It is very disadvantageous for Bethesda to do so, because people will put it on Ultra and then complain on various forums that they have problems with the framerate.
Setting graphics to Ultra leaves God Rays to High FWIW.
I also don't really think it's that taxing seeing how a 2.7GHz - 3.5GHz Sandy Bridge + stock 2 yo 780 Ti run it effortlessly at 1200p. Whether the setting actually adds significant eyecandy, I don't know. Haven't turned it off to see what the difference is, but it runs fine with it enabled so I don't see the point of disabling it.