The farm in Oklahoma provides a sense of identity to the Joads, as homes usually do for the families that live in them. A home allows a person to say, "I belong here, I am from here, this is my place." Once the Joads are stripped of the farm, they are wanderers. They mix in with other migrant workers, becoming part of a larger family of Americans, but losing their own sense of family as they do. They have no place to which they belong and can return, and have only each other to cling to. Steinbeck proves the inadequacy of this last truth by separating the family more and more the farther away from Oklahoma they travel (Granma's death, Tom's exile, etc.).
The loss of family identity is reinforced at the novel's close, when the remaining Joads take refuge in a new "home", the barn. Here Rose of Sharon suckles not her own newborn, but an ailing man, thereby extending her family to include the other migrants and showing the Joads to be part of a larger whole.