Verbal commands are frequently an under utilized self-defense tool. Verbal commands can be applied early, without liability, and throughout the encounter in conjunction with other levels of force.

Remove Ambiguity. Issuing verbal commands can help you determine if someone is really a threat. A stranger approaching you in a dark parking lot might be perfectly innocent, but the potential for danger is too high to make this assumption. The circumstances are not in your favor.

If you yell "Stop! Don't come any closer!" most normal people would probably stop. To continue approaching someone who is telling you to stop is threatening. To approach faster is very threatening.

Create Hesitation. Getting a command to stop will force many people to hesitate, especially if they haven't committed to a course of action yet. That hesitation buys you a little bit more time to escape, prepare to fight, or further evaluate the situation.

Provide Direction. Tell the assailant what you want him to do. Let Go! or Go Away! in conjunction with another level of force tells the assailant how to avoid getting hurt (or getting hurt any more).

If they are now facing your drawn handgun they might not know what to do - the problem has moved off of their mental map sheet. We don't want the assailant to 'freeze' like on a bad TV show, we want him to run as far and as fast as his little criminal feet will carry him.

Alert the Witnesses. Many crimes need to happen in isolation. If you can draw a crowd you might be able to prevent the attack. Maybe there is someone that will come and help us.

At a minimum we are establishing who is the defender and who is the aggressor. This sets the context for all our other actions in the mind of the witnesses. If we have already shouted Stop! Stay Back! and the assailant has closed the distance then we have a justification for using another level of force.