Sometimes people use anger as a way to get others to react in a certain way.

 For example, they want the other person to do them a favor and if the other person hesitates they get mad so the other person will do what they want. Or they want to gain control over the other person and they know if they get angry the other person will back down. Or they want to make the other person afraid of them. Sometimes people even use it as a way to get out of doing things. Someone asks them to help them with something and they get mad at the other person not only to get out of the current situation but to discourage the other person from ever asking again.