It is necessary to forgive so that you are not weighed down or in pain. You need to forgive so you can move on. Unforgiveness breeds anger and bitterness - it ends up hurting you - not the person who offended you.
When you hold on to anger toward someone what you are actually doing is giving them power over you. You give them the power to make you angry. Don't do that. Don't allow anyone to have the power to disrupt your life.
I had to learn this lesson myself. A very wise and loving friend taught me the concept of forgiveness - whether or not the person deserves it - is not the question!