The answer is one very important word: closure.
Although underestimated at times, closure is the final and the most transcendental moment that completes the grieving process. It is a most courageous step to take: the act of finalizing pain and moving on. However, in The Lovely Bones closure goes amiss. This is because the pain of Susie's death has left such a deep dent in the psyche of the Salmon family that their grieving process seems almost interminable. However, it is through this process, and only through grieving, that closure can actually happen.
Grief takes center stage in The Lovely Bones, as the agent of change in the novel. It is grief that defines each character, and their reactions to grief color them with realism and intensity. Grief is the motivation behind the most important life decisions made by Susie's mother and father, as well as the rest of her family. However, what makes The Lovely Bones such a wonderful study in human dynamics is that it is one of the few literary works which explores grief as it occurs outside the nucleus of pain.
Ray's character is significant because he represents Susie Salmon's essence of youth: the first crush, the ideal boyfriend, the first signs of womanhood. Yet, when Susie is murdered, not only does her life becomes cancelled in the real world, but also in the afterlife; Susie's spectral self remains, even years after her death, as a 14 year-old girl. Her unfinished dreams and stranded development remains with her as well. Hence, the final moment with Ray will bring closure to Susie's own grieving process: she grieves for her lost life, and her missed opportunities. Now, she has had the one thing her 14 year old soul wanted: to be with Ray.
Conversely, Ray also suffers grief after Susie's death. Although he does not express it dramatically, he did have an emotional bond with her during her life. He also grieves the injustice of her death and the lack of justice brought to Susie's perpetrator. Moreover, he also grieves (like Susie) for the lost opportunities, and for the "what if's". Regardless of whether he has moved on with his life, he never gets to have the closure of knowing what would have happened if he had the chance to see Susie again. This final moment brings him the closure that he deserves as well.
Finally, this event also gives closure to Ruth, who accidentally became Susie's medium after Susie's spirit brushed up against Ruth shortly after the murder. Ruth, a loner and a woman very much like Susie, has always struggled with her own identity; she constantly contemplates life, and her role within society. Susie was for Ruth a reason to find a purpose. However, Ruth has grown up at this point and perhaps now understands that, like Susie, they both have to "let go" and move on. By serving as Susie's channel to connect physically with Ray, Ruth is giving Susie life; this is proportional to the life that Susie seems to have awoken in Ruth when she connected with her cosmically. Therefore, Ruth, Ray, and Susie literally complete the circle and bring closure to the life and death of Susie Salmon.