Well I can give you a reason for why not to let anger simmer (not letting the sun go down - a day pass by - with unresolved anger). The reason is because it leads to bitterness which gives Satan a foothold to lead us into a multitude of other sins. Hebrews 12:15 says, "Look diligently lest any man fall short of the grace of God: lest any root of bitterness springing up trouble you, and thereby be defiled". The root of bitterness starts in a man's heart, and anger and rage and jelously and other excessives of the human heart if not dealt with quickly will lead into sin (James 1:14).
I don't have a lot of time to go into it all now, but did that answer your question, atleast partly?
God Bless,
~Josh