Because normal voice wouldn't near show the intensity necessary for some things.
Saying growls are bad because they have no melody is like insulting the drums for having no melody, and requesting all bands use xylophone instead.
Where am I, at the Linguistics Cafe? Because everybody here is putting words in my mouth!
I never, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, EVER said that the reason growls were bad was because they don't have melody. (I never even said growls were bad. They're not good or bad; they're just growls. I said I found them silly and I would prefer Akerfeldt to always sing cleanly.)
If we're going to have a discussion on this topic, we're going to have to learn to make relevant arguments.
EDIT: ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever, ever,