Rage is the pinnacle of the darker emotions in life. As most of us have experienced, emotions lead to other emotions. Therefore, rage can be construed as the culmination of sadness, anger, frustration, and even hate.
Rage in and of itself is dangerous. People who cannot control it will usually destroy everything around them without even a thought. When a person is in the grip of rage, it's as if he or she has devolved into something primal, something that can only react like an animal. I've personally hurt those around me because of this dark emotion.
If someone can control their rage it can be extremely useful in life. I use my rage whenever I work out at the gym. I tap that inner store of rage and immediately begin to feel the adrenaline and endorphine rush that is induced by the emotion. Soon, I'm done with my workout and far more exhausted than most others around me.
I guess that, in closing, we as people should learn to analyze our emotions and find out why we feel them. By using this method of self-analysis we can supress our emotions and learn to invoke them later on when needed. It can be a dangerous path to follow, possibly leading to the devaluation of your emotions, but it can also bring more self-control and discipline to your life if you make the effort.