Although we practice kiai in rowing exercises at the beginning of class, we don't usually kiai during techniques in our dojo; however, I went through a period where I did. If someone much bigger and stronger came at me fast, it just seemed to happen. And somehow it helped me MOVE and keep my posture. I don't know; maybe it shut my mind up for a few seconds and let my body react. Now I don't normally do it, and I don't know why that happened either.
We do that as well. And we do it to ensure familiarity and comfort in performing vocal kiai, but never insist that it be used or not be used in a given setting. All the same, it is important to train in doing it, both for its primary uses, and for its collateral effect in sensitizing the internal rhythm and shape of movements.
I wonder what anyone thinks of the more projective "blood-curdling" aspects of its uses -- East or West.