Anger energizes
Anger arouses parts our brain that give us a surge of energy. This energy can motivate us to act. Anger and fear are essential emotional resources for coping with danger. Danger (note the embedded word anger in "danger") can be either a threat to safety or a perceived injustice.

In the abduction, we could display a burst of strength to resist the efforts of our attacker.

In the irresponsible act by the teacher, the anger can give us the motivation to oppose that behavior to ensure it does not happen again.

In the billing mistake, anger motivates us to rise from our seat to contact the company to insist on a correction.

With the exception of the first situation, the anger is controlled and specific and is accompanied by proper corrective action. In other words, we were not going berserk and acting unreasonably.

Let's look at a second reason why anger is sometimes helpful.