They hurt people, and the hurt is very real. The Jews of old saw sin as a debt to God. Sin is pain to all involved, the offender, the forgiver and God. A forgiving attitude makes the healing of pain easier for all. God has that forgiving attitude, and so can we.
Good things usher in joy, bad things bring on pain.
And pain separates people. Offenses divide people. They no longer feel comfortable together. The offender causes pain, the forgiver receives that pain. If there is no forgiveness, the pain just bounces endlessly between the two, multiplying as time goes on.