Walking around with unexpressed anger and upset generates greater ire. Anger feeds on itself, becoming stronger and more perilous. Finding ways to process the small irritations and the larger aggravations leads to a physically, emotionally, mentally, and spiritually healthier you.
Anger can serve a purpose too, when it serves to motivate us toward action, but when it is buried and denied, it WILL come out, usually in explosions (like volcanos).