Why do people yell when angry?

People yell when they are angry for much the same reason that dogs bark. The loud voice is a threat. It says, I am angry, and I may become violent, so you had better listen to what I am saying. Loud voices also have the advantage of drowning out quieter voices. Angry people do not want to listen to what other people have to say, they just want to be heard, and preferably obeyed.