Sonya was the only person who could help Raskolnikov change. She listened to him as he told her that he murdered, and she didn't even leave him after finding out that he killed her friend, Lizaveta. She followed him to Siberia to show her dedication. Although moving to Siberia for many years seems extreme, it shows Raskolnikov that someone will always be there for him. Just seeing support helps someone recover and change. Additionally, she brought about his gradual conversion, and helped him to become more rational. Her symbolic passing of the cross showed her commitment to Raskolnikov. If Raskolnikov had not had Sonya in his life, he never could have had the strength or rationality to consider religion as an outlet for his sins.
Similarly, Sonya needed Raskolnikov to escape a life of destitution and prostitution. He could support her to get out of the career that she was in. She even made the leap to owning her own sewing business. Granted, her changes were more subtle. She still depended on men to take of her and never became an independent woman. She also ended the book as a major character that we know little about. Her beliefs were strong, she was sometimes weak, and she cared about Raskolnikov were about all that we knew.
Together, the two were better off than alone. They had to lean heavily on each other to make any changes in either of their lives. They helped each other find their own versions of "truth" and what is good. Even without a strong bond of love, even in a relationship of convenience, these two made sense.
Do you believe that Sonya and Raskolnikov love or could ever love each other?