"Although she appears in just two scenes, her influence is evident throughout the play." She foreshadows events in the play. Queen Margaret curses her fellow royals, as the wife of the dead King Henry VI, she is left without a husband to protect her, she is at the mercy of her family for charity in order to survive. Queen Margaret speaks the rage that she feels toward Richard as a spokesperson for other characters he has wronged. She draws our focus to Richard's evil.
First she reminds us why she hates Richard.
"Out, devil! I remember
them too well:
Thou kill'dst my husband Henry in the Tower,
And Edward, my poor son, at Tewksbury." (Act I, Scene III)
Then she curses him for his grabbing desire of power, wishing on him the inability of knowing his friends from his enemies.
"On thee, the troubler of the poor world's peace.
The worm of conscience still begnaw thy soul!
Thy friends suspect for traitors while thou liv'st
And take deep traitors for thy dearest friends!
No sleep close up that deadly eye of thine,
Unless it be while some tormenting dream
Affrights thee with a hell of ugly devils!
Thou elvish-mark'd, abortive, rooting hog!
Thou that wast seal'd in thy nativity
The slave of nature and the son of hell!
Thou slander of thy mother's heavy womb!
Thou loathed issue of thy father's loins!
Thou rag of honour! thou detested" Act I, Scene III)
Historically, Margaret was the widow of Henry VI, who died while in the custody of Edward IV. In the civil war between the Lancasters and the Yorks, Henry VI was a Lancaster, and Edward IV was a York. Poor Henry was the son of the great warrior king Henry V, but he inherited the mental illness that many of his mother's relatives (most notably her father) suffered from. Most scholars believe that he was murdered, but there is no evidence that Richard III was the killer.
Historically, Margaret returned to France after Henry's death. In Shakespeare's play, however, she stays in England and represents the Lancaster cause. As the eNotes study guide tells us, she acts as the "chorus" in the play, "offering her opinion on the play's action, and prophesying doom and misery on Richard and his supporters."
By keeping the Lancaster memory front and center, Margaret also lays the basis for the Tudor claim to the throne. Henry VII, the first Tudor king and grandfather of Shakespeare's patron Elizabeth I, was the grandson of Henry VI's mother, Katherine of Valois.
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Queen Margaret's foreshadowing and prophecies are essential to the play. In the first scene that she appears in, in cursing all the courtier, she basically outlines the plot of the play. Each of them dies/suffers in the order that she predicts. The importance of her curses is further increased when each character recalls her prophetic words at the time of his death. Eg) Buckingham, Stanley.
Margaret is also one of the three prominent female characters. The women in the play have an important role. They illustrate the devastation that Richard has caused through their melodramatic speeches and hysterics. Their state in the play also reflects the position of women at that time in society - how helpless and submissive they are. Margaret stands up to the male characters, but at the end of the day, she can only curse them and has no other means of attack.
She is also the only character we see from the House of Lancaster. The play has historical context to it, and in that, she is important to link the past and present of the play.