In Chapter 17, Book 2, what is the significance of Lucy and Dr. Manette's conversation?

The night before Lucie's wedding, she and her father sit beneath a tree in their yard, and he tells her how happy he is about her marriage. He relates to her how, in his suffering in the Bastille, he imagined his daughter as a specter, comforting him, showing him her children, remembering him. She then tells her father that her marriage will never separate them.