I am in martial arts and the people here got it pretty right, it varies from style to style but the reasons for it are many. First of the word hiya is not a necessity, it is any word you can form into a loud short yell, (I have heard HEI, ICE, DI, KI, KISA ect.) it is used as an empowerment, to make you look strong, and also it is to release all the air from you lungs as you are struck so your ribs don’t break if you get hit (that is why you normally give a kiop during a technique that leaves you slightly open)