One is defined by what one does, and Madeline Elster and John Feguson are defined by their lack of direction: they "wander." "There's nothing you must do," and nothing is what all three major characters devote themselves to.
To be aimless is not to avoid an end, but merely to miss the chance for an intended end. "Two are always going somewhere," and if they don't go "necessarily," then their slow drifting float will eventually lead to a slippery ride down the falls: "The mission? That's Skid Row, isn't it?"