Why does the narrator call attention to Robbie’s, Nettle’s, and Mace’s acts of kindness?

Atonement is a 2001 novel written by Ian McEwan. Early in the novel, the narrator, Briony Tallis, witnesses her sister, Cecilia, and Robbie sharing an intimate moment in the family's library. She believes that Robbie is being sexually aggressive toward her sister, so later in the novel, when Robbie asks Briony to deliver a letter to Cecilia, she reads it to find an explicit confession of Robbie's true feelings for Cecilia. Because Briony is only a child, she doesn't quite understand what the letter means. She is confused by Robbie's confessions and is not keen on his relationship with her sister.
Later that evening, when the twins go missing during dinner, Briony discovers her cousin Lola being sexually assaulted during the search. Although Briony did not clearly see who committed the crime, she thinks back to the scene in the family's library and tells the police that it was Robbie who raped her cousin. Despite his innocence, the police take Robbie away.
As the novel follows Briony's life as a writer, she learns more about life itself, which includes the situation that took place that fateful night at her family's home. She begins to understand that Robbie was not only innocent, but that he truly loved Cecilia. Throughout the book, she mentions others' acts of kindness toward her. She believes that writing the story of Robbie and Cecilia's life is her way to atone (hence the title of the book) for what she had done as a child. For example, in the epilogue, Briony mentions how the letters Nettle sent her helped her recreate Robbie's trip to Dunkirk, which further developed the story she was writing.
At the end of the novel, Briony states, ”I like to think that it isn't weakness or evasion, but a final act of kindness, a stand against oblivion and despair, to let my lovers live and unite them at the end. I gave them happiness, but I was not so self-serving as to let them forgive me." Briony is speaking of Robbie and Cecilia, who in her version of the story end up living together happily. At this point in the novel, Briony is now an elderly woman who truly understands the extent of the damage she caused by lying to the police. Although she has written a life where Cecilia and Robbie end up together, she knows this is not the reality her sister and Robbie faced, thus the latter half of the line. Briony considers Robbie, Mace, and Nettle's acts of kindness, recognizing that they were kind to her when they had no reason to be.