Peace Like a River, by Leif Enger, follows Jeremiah Land and two of his children (Reuben and Swede) as they search for the oldest son, Davy. They have all lived many years without an adult woman in their lives, so when Roxanna becomes part of their family, everything changes. In the broadest terms, what Roxanna provides for the Lands is joy.
Roxanna knows how to have fun, and she is able to do that even under difficult circumstances.
She knew...that when the land lies buried in the miseries of winter, and loose boards twist with the cold, and the air hangs glassy and absolute over the world, that is when the merriest of all picnics happen.
Though all of the Lands do have a sense of humor, they are all rather serious-minded and do not really know how to just have fun. This is a gift Roxanna's presence brings them.
Roxanna also brings her new family joy through her beauty. She is physically beautiful, but she has "a low, beautiful laugh, and hearing it you could only wish you'd said the thing that brought it forth."
Finally, she brings a sense of completion to the Land family. Though none of the Lands has ever met anyone like Roxanna Cawley, she fits into all their lives as if she has always been there. When Jeremiah has to wake Reuben from a nightmare which triggers a breathing episode (an unfamiliar experience for Roxanna), Reuben says "she pounded [my back] as if not just mine but both our lives were reckoned by her strength."
Just by being herself, Roxanna brings joy to every member of the Land family in ways which are most meaningful to them. When Jeremiah is gone, his children will be with a woman who will bring them joy, even in their sorrow.