"Cancel his bond of life, dear God, I plead. / That I may live and to say, 'The dog is dead'" (4.4.7). In "Characters: Queen Margaret," we told you that the old queen is like the Olympic champion of curse hurling. As you can see here, we weren't kidding around. When Margaret prays for Richard to be punished for his treachery, she asks God to execute divine justice. So later, when Richard is killed in battle (5.8), we get the sense that God really has answered Margaret's plea and that Richard's death (along with Henry VII's establishment of Tudor rule) has been ordained by God.
This isn't the only example of Margaret's curses coming true. The old woman sees just about everything coming, which is why her curses are considered prophetic. It also doesn't hurt that we (as good students of history) pretty much know how things are going to turn out. In other words, the play's historical foreknowledge helps establish the sense that events unfold according to some divine plan. (Plus, it suggests that when people get murdered and done away with in the play, they were basically asking for it.)
Also, every time a curse is fulfilled (even if we knew it was coming) the audience gets the satisfying feel of a real narrative at work. It's as if the curses give us some dramatic tension to look forward to. We as the audience know that the curses are serious business, so we also enjoy knowing the fate of the curse's victims before they do. That's the definition of dramatic irony – keeping the audience engaged with some secret that the characters don't know. Without the curses (and the fulfilling feeling they give us when they come true), the play would really just be a long boring string of deaths and executions that we knew were coming.
Curses serve a dramatic theatrical function. Remember that Shakespeare's audience was subject to the Tudor Queen Elizabeth I, and this is their national history that led up to her. They already know the whole story – for them not to know that Richard's defeat was inevitable would be like us watching a movie about JFK and being surprised when he was shot at the end. The curses of the women (and the ghosts, Rivers, Gray and Vaughn, and Hastings) all lend a mystical aura to the play.
Margaret isn't the only character who likes to curse people. Actually, all of the adult women in the play curse Richard at some point: Lady Anne curses him (and herself) over her father-in-law's corpse (1.2), and the Duchess of York curses her son by asking for his bloody death (4.4). Interestingly, Elizabeth begs Margaret to learn how to curse (4.4), but she admits that curses are a little unsatisfying. They are, in reality, just empty words that often vent anger and soothe the heart. Of course, the irony is that the women's curses are much more than windy words. The women curse because they feel powerless, but their curses actually give them power, as all of their curses come true.
We don't want to be redundant, Shmoopsters, so if you want to know more about all this cursing business, go read what we have to say in "Characters: Margaret."