Anger provides a mixture of motivational benefits, some healthy and some short sighted and self-destructive.
On the positive side, anger creates a sense of power and control in a situation where prior to anger these positive, motivating feelings did not exist. The feelings of control and righteousness that come from anger can motivate you to challenge and change difficult interpersonal and social injustices. If handled correctly, your anger can motivate others to help you win your cause. Anger can provide you with a rest from feelings of vulnerability, and a way of venting tensions and frustrations. It can provide the energy and resolve necessary to defend yourself when you've been wronged. If you are a long suffering victim of domestic abuse, for example, and your anger finally reaches the boiling point so that it enables you to leave your abusive relationship, anger has been a truly positive force in your life. If you are a dedicated crusader working to further a truly moral cause (such as Martin Luther King Jr.'s drive for civil rights, or Gandhi's drive for Indian independence), then anger gives you the strength to carry on, and the will to persevere.
There are negative motivational sides to anger too though. Anger can create and then reinforce a false sense of entitlement, a feeling of being better than others and deserving better things, that can be used to justify immoral actions. For instance, anger-motivated aggression can be used to justify terrorism, or to coerce and bully people into doing what you want them to do against their will. Angry people are likely to believe the philosophy that "the end justifies the means" and then use unspeakable means of working towards their goals that defeat their purpose. If you are a terrorist like Timothy McVey (who bombed the Oklahoma City Federal Building in 1995), a bully like television's Tony Soprano (lead character in the HBO drama “The Sopranos”), or a 'school shooter' like Eric Harris and Dylan Klebold (who murdered fellow high school students in Columbine, Colorado in 1999), anger has led you to the dark side.
It is important to recognize that the effect of anger can be either positive or negative. If years of unresolved anger reach the boiling point and motivate you to leave an abusive relationship, your anger has saved you from additional abuse. On the other hand, if you use your anger to frighten others into doing what you want them to without considering their needs, you are allowing your anger to coerce and control others and you are no better than a bully.