Projecting Anger

I find it interesting hearing about people projecting their anger and emotions towards someone, when they were given it by someone else. I can understand being frustrated, but why hurt someone else when you can hurt the main cause of your distress and anger? Personally, if someone wrongs me I just remember that they wronged me, and exact revenge either at the time or much later. I can understand projection but I don't see how it can become sustained projection, rather than just an outburst.