What do the frame narratives accomplish in de Navarre’s The Heptameron?

Basically, a frame narrative or story is defined as a story within a story. In Margaret de Navarre's The Heptameron, frame narratives highlight God's providence and man's reliance on divine care. They also highlight the complexities within relationships and the therapeutic effect of stories during times of trial.
There are 72 short stories in The Heptameron; the narrators of the stories are five male and five female aristocrats who have been stranded at the Notre Dame de Serrance monastery after a flash flood. They must wait for a bridge to be rebuilt before they can continue on their journey. In the meantime, the lively group tells stories to amuse themselves. Navarre's narrators use diegesis (both homodiegesis and heterodiegesis, where all speech is filtered through the voice of the narrator) to tell their stories. In homodiegesis, the narrator is a character in his/her story, while in heterodiegesis, the narrator is outside the story. Navarre also allows her narrators to utilize independent dialogue to highlight the voices of characters in the story.
Let's take the first story on the first day. This story is told by Simontault, and it highlights the treacherous nature of immoral women. Simontault himself is not a character in the story; he allows his characters to speak for themselves. The use of heterodiegesis and dialogue in Simontault's frame narrative is followed by Oisille's frame narrative about women's virtue. Both frame narratives allow Navarre to draw attention to dual 16th century narratives regarding female morality.
While Simontault argues that "all women have set themselves to bring about the torment, slaughter and damnation of men" ever since the days of Adam and Eve, Oisille prefers to draw attention to female virtue. She tells the story of a virtuous wife who refuses to sully her chastity when she is propositioned by a licentious servant. Oisille's heterodiegesis at the end of the story highlights woman's pious nature, but it is also Navarre's means of drawing attention to her personal brand of theology. Navarre championed a stronger version of Calvin's "Singular Providence," where worshipers who submitted themselves to God were inspired to live singularly moral lives. In Oisille's story, the wife of the muleteer submits to twenty-five "mortal wounds" from her servant's sword rather than acquiesce to his base intentions. She remains faithful to her moral dictates.
In the end, the muleteer's wife is so weakened from loss of blood that she becomes helpless when the servant ravishes her. Oisille proclaims that the pious woman dies placing "her hope of salvation in Jesus Christ alone" and that she dies with "glad countenance and eyes upraised to heaven." Besides highlighting the widely divergent views regarding feminine piety, Navarre also uses The Heptameron's frame narratives to epitomize the highly complex nature of male/female relationships. For example, the stories on Day Six draw attention to how women and men often deceive and entrap each other through acts of vengeance and moral turpitude.
Last but not least, Navarre's frame narratives highlight God's sovereignty in all matters pertaining to the human world. In this vein, even treacherous weather must be attributed to God's will; it highlights his position as the only one who can bridle the natural world's active chaos. In Navarre's The Heptameron, the narrators are sheltered in a monastery, symbolic of God's protection against the elements. The aristocratic group wait out the storm and amuse themselves by telling stories. Navarre shows that floods and catastrophes are cause for thanksgiving because they bring to light God's continuing mercy upon humankind. It is God who preserves humanity and delivers them from sundry natural disasters.
To recap, Navarre's frame narratives highlight God's providential care for humanity, the complexities inherent in male/female relationships, and the therapeutic effect of stories during times of trial.
Sources:
1) Authorial and Narrative Voice in The Heptaméron by DEBORAH N. LOSSE Renaissance and Reformation / Renaissance et Réforme New Series / Nouvelle Série, Vol. 11, No. 3 (Summer / Été 1987), pp. 223-242.
2) The Pleasure of Discernment: Margaret de Navarre as Theologian by Carol Thysell.