what is the significance of the letter Rahim Khan wrote to Amir before he leaves Peshawar?

After Sohrab left, Amir read Rahim Khan's letter. In it, he revealed that Hassan told him about the rape soon after it happened. He told Amir that he did betray his friend, but reminded him that he was only a boy at the time. He assured Amir that he had suffered from his guilt so much only because he was a good, caring person. Rahim Khan explained that it was hard for him to watch Amir vying for Baba's attention. Baba, he said, was hard on Amir only because of his own guilt. His betrayal of Ali and the fact that he could never claim Hassan as his son tortured him. Rahim Khan believed that all of Baba's charity was in atonement for his sin. Amir, he said, should learn from Baba's example and try to redeem himself as well. He said he had left money for Amir in a safety deposit box, which the key would open. He ended the letter by requesting that Amir not look for him. Amir cried reading the letter. He was ashamed that, unlike Baba, he had acted out because of his guilt rather than doing good.