This is not about buying drinks for friends in a pub. This is the yell made by martial artists during training called KIAI. There are many purposes for this, but to begin with, it is when one focuses all the energy to one particular strike (or any other movement) with explosive power. Another, is to control the breathing at the moment of impact. A third reason is to distract or even intimidate your opponent. But it doesn’t end here if we start talking about the spiritual side.
A point which is often overlooked is the psychological affect it has on you, the person doing the KIAI. In ancient battle times lines of soldiers running at each other with crude weapons, when some were too scared to fight, would yell at the top of their lungs as they charge the enemy. This shout of courage can bring out the animal instinct within to address the "fight/flight/or freeze" syndrome.
KIAI is a valuable tool for the Instructor. When I ask students to KIAI I am asking them to fully focus their mind and energy on a particular strike. If I get a poor response from anyone it is very likely the student is not focused during the entire training session. This is wonderful feedback for the Instructor. I assess students by their reactions to the KIAI command ie who is focused on their training, or not, and then I can do something about it. Also, the KIAI assists the student to stay on track or fully reengages their attention if off target.
A weak KIAI can reveal a lack of confidence or spirit in a student which could give the Instructor the heads up on such person . The Instructor should use the KIAI command seldom so it will not lose the effect it has on the student. Furthermore, too much KIAI, during a sparring match will also lose the effect it may have on an opponent.