Kiai

Myomoto Musashi, in his The Book of Five Rings, said there were three types of kiais:

• Pre-emptive. Used to scare the opponent or draw him off his guard. Also used as a way for a warrior to release his own fears as he went in for the kill.
• In-fight. Acts as a psychological attack and gives added power to physical attacks.
• Victory. Serves as a release of pent-up energy and as a warning to others who would attack the warrior.

As a student of Taekwondo, you learn how to kiai properly so it reinforces your technique. You also learn to not flinch and freeze at sudden loud noises so you will be able to more quickly assess the source of the noise, whether it poses any threat, and what, if any, response is required.