Kendo is a Japanese martial art with a tradition of ki ken tai icchi, meaning "spirit, sword and body together." Part of that is screaming, which may have several purposes. Scientists have recently been looking into benefits of making loud sounds in elite competition like tennis:
When the impact of a grunt is investigated, there is evidence that hitting performance is enhanced. Skilled university tennis players were found to hit with a 3.8% increase in groundstroke hitting velocity when they grunted.
For a serve, a 4.9% enhancement in velocity was found among players who grunted. This translated to "grunted serves" being hit 7km/hr faster than those that were not.
While the increase in hitting velocity came at no additional physiological cost, in relation to perception of effort and energy consumption, there was an increase in force production as measured by muscular activity. Overall this suggests that grunting is performance-enhancing, and is a sustainable strategy over the course of a match.
Australia's ABC also looked into why do they scream so much in kendo?
Here's another competition with slow mo replays that demonstrate the incredible reflexes of the kendo competitors.