I agree with you - how does anger helps us? If we despise someone, who takes the damage from it?

Since, today, anger usually remains inside the one who is angry (you cannot simply take a club and smash someone’s head like you could a couple of millennia ago) and the despised person often doesn’t even know about it or doesn’t care about it, the one who is angry is actually the one that gets hurt from his/her own anger towards someone else.

Thus, it's great that you forgave and move on - not for someone else’s sake, but for your own sake!