Margaret is one of the only characters who appears in—and lives through!—all three Henry VI plays and Richard III. But she's often cut from modern productions of Richard III—I think largely due to time constraints.
The Hollow Crown's version is one exception. And that means that their version can explore in greater detail the curses she utters early in the play and their effects through the play.
Here's a clip that shows those curses—and also the way Margaret serves as a common enemy to the otherwise squabbling royals.
Links: The Film at IMDB.