In my opinion, it is good because God wills it.
(feeling my way here ...)
Then we have to ask if "good" is just another word for "God wills it." Is there an idea of "good" that is validly or usefully different from the idea of God.
The OTHER side is: How can we say God is God if he must conform to a standard of Justice, Goodness, Beauty, etc. Is that making some concept(s) or "ideas" or "forms" superior to God? (which is absurd)
Lewis works the problem in Mere Christianity, I think.
It has to do with reason and especially with the fallibility of human reason. It's not USEless, but it's certainly imperfect "for now we see only in part."
I think it also leaks into questions of conscience. If somebody tells me that God wills that I blow up a plane full of mothers and children I don't need special revelation to know something is wrong with that.