here is how I see it about the harsher vocal styles...

For Brutal Death metal, the concept was to be basically merciless, destructive,violent, disgusting etc. When I think of these I can think of a demon or something similar in like a horror movie, and listen to how they have their voices, the demonic low scary type of things. they represent those words in the movies and thats how I think the vocals are supposed to be portrayed


For Screeching Black Metal, it is used to represent the anguish or pain etc of the music, and when a person is in the most extreme form of pain and anguish, they usually well.. screech