The purpse of yelling (aka "kiai") when performing a technique is firstly to clear the mind of any other thoughts and secondly because a technique is apparently 40% more powerful if you vocalise while executing it. (Olympic weightlifters I have seen on TV also yell when snatchnig/cleaning). It's also a normal part of martial arts training to get the students to kiai a lot because it improves confidence and increases aggression. Eventually it becomes automatic, execution of a striking technique -> kiai. And it also makes stage performances more "fierce". It doesn't intimidate anyone during sparring as it's such an integral part of the practice.