This is an interesting question. I am assuming that you are asking about the positive effects of anger, because you know that anger for the most part is not positive. With that said, there are a number of positive points of anger.
First, when there is injustice, there should be anger. I would call this righteous anger. It is the anger that emerges when there is evil, such as when the rich exploit the poor or when one group oppresses another group. In addition, there should be anger over discrimination, sexism, and the like. From this perspective, we might even say that the lack of anger is wrong.
Second, this anger can be a powerful catalyst for action. To put it another way, anger is a powerful force that can move people into doing something about the injustices of the world. For this reason, it is good to stay angry, but the key is to stay angry for the right things.