I believe that a strong kiai or "war face" helps direct/channel your energy or adrenaline. Couple that with a strong attack or charge and it can be quite intimidating.
That being said, I personally don't use one because I haven't developed a really good one yet. My exhales (giving or receiving) are more along the lines of the muay thai "yeeeshhhh". It may be psychological, but when I'm really whaling on the bag or pads, loud exhales help me focus.
As you guys know, some type of exhale (loud or quiet) is necessary on every punch/kick. When executing a combination of strikes, most beginners hold their breath through the whole combination, thereby unnecessarily fatiguing themselves. If you can remember to breathe out, your body will automatically breathe back in for you.
If you are having trouble taking a body shot while inhaling, the following drill may help. Stand straight up and put your hands behind or on top of your head. For a round, allow your partner to throw blows (vary the timing and target) at your body/torso (careful to avoid the solar plexus) while you take long slow regular even audible breaths in and out or carry on a normal conversation (you can stand with your back against a wall or you can slowly move around). Your goal is to be able to keep your stomach muscles tight for several minutes, while breathing and talking normally. Your partner's goal is to catch you if you relax. Make sure you are in agreement on the intensity level and up it when you are comfortable.
You can practice the stomach tightening by yourself anywhere (in the car, at work, at school, etc). Even in the mall, tighten up and stay tight whenever someone is in your "red zone" and when they're gone, relax until the next person walks by.