Don't let anger get the best of you

A lot of people especially parents say don’t let anger get the best of you. Why is this? I believe that once you get angry you don’t think about what you are saying and shout out whatever the first thing that comes to mind. Often times this will be worded in such a way that you will end up saying something you don’t mean to say or saying it in a way you don’t want to say it. I am guilty of this and have not talked to my friend Jared since.
One day me and Jared got into a huge argument over something, I can’t recall exactly what it was. The main thing I remember is I knew that I was right because I had done research on the topic and he was not fully informed on the topic. He was unwilling to change his opinion or look at the computer so I could prove him wrong. It turned from an argument to shouting in a matter of minutes. I started verbally assaulting my friend by calling him ignorant and close minded. Had I been more gentle in my approach I am sure that we could have come to a non friend threatening argument. To this day I have done my best to think out what I am going to say and stay calm when I argue with someone because people have different reactions to different styles of argument.