Most of the things that make us angry do not threaten our personal physical existence. Most of them do not require the burst of adrenaline that drives through us when we are angry. Most of them are better handled with a cooler head. So why do we still get angry?
Like pain, anger signals all parts of us that something is wrong. It also signals those around us that something is wrong. Anger expressed is a kind of collective pain: a signal in a group or relationship that something is a threat to well-being and integrity. When we ignore anger (whether our own or someone else's) we are, in effect, doing the same thing we do when we turn off the smoke alarm while grilling something in the kitchen.
We do remarkable things when we ignore warnings and change well-established patterns. Some of these things are useful.