and some more thoughts...
By holding onto anger you are letting that person or event linger inside of you. In essence allowing that person to continue hurting you. You are being 'controlled' by that person - you are designating time and space for that memory to continue to dwell in your todays and to affect the way you feel.
It is like 'leaving that door open' and letting that action reach from the past into your present. Like 'living with the enemy' or 'walking with a rock in your shoe'. Wouldn't you want them out?
It is empowering the hurt to still have a place in your life.
Sometimes it may even be ourselves that we need to forgive, for not being or knowing better or reacting faster or protecting ourselves or our loved ones or being unkind to a person we can no longer make it up to or whatever it may be. We also have to be able to forgive ourselves to release our spirit to grow and be free.
I always think of (the gist of)that saying: "Hatred is like taking venom and hoping that the enemy will die."
It kind of illustrates 'why forgive' pretty well.
MadArt (ist)
PS: Forgive not to be confused with forget.