The Kiai--that shout associated with a karate blow--is important. When you deliver a forceful yell, you exhale. When you exhale forcefully, you generate power. This energy unifies you with the moment. You are focused in the present. There is no past. There is no future. There is only-Now.
However, just yelling is not a kiai. If your throat hurts, then you've only tapped into the Tip of the Iceberg. This writing is not a How to. However, ask a parent or a teacher how they got that shout that can Stop-You-in-Your-Tracks.
There are many Western mantras that help people organize their thoughts, body, and their mental focus to a purpose. For example, there's, "Hallelujah!" "Yahoo!" "Heehaw!" If you're a Southern Baptist you happily shout out, "Praise the Lord!"
The shout, the kiai, is inspirational and motivational. It encourages and empowers.
In many aikido schools, the kiai is not used. However, O'Sensei, the founder of aikido, had a mystical kiai himself. I read that O'Sensei was on a bus that was on a collision course and at the moment of impact his kiai flipped the bus safely back onto its wheels.
The kiai has to have a purpose. At first, one is drilled into kiais. However, after that phase, one just knows when to execute it. Although, I have seen some black belts delivering kiais for no apparent reason such as those in the aikido style that kiais loudly just for grabbing the wrist-- ridiculous.
Be that as it may, a well placed kiai can stun an opponent without one ever having to touch'em. The kiai is worth further study.
Check out the first 38 seconds.