Rebecca Nurse is the quintessential good Puritan woman. In fact, she is so good that her reputation precedes her. When Mr. Hale arrives at Reverend Parris's residence and sees Rebecca, he knows her by sight though they have never met. He says, "It's strange how I knew you, but I suppose you look as such a good soul should. We have all heard of your great charities in Beverly." Rebecca's presence is so peaceful and calming that it and her gentle touch actually quiet Betty. Later, when Rebecca has been arrested, Hale says, "if Rebecca Nurse be tainted, then nothing's left to stop the whole green world from burning."
Rebecca is a good Christian: loving, kind, accepting of difference, compassionate, and optimistic concerning her fellows. She is the yardstick by which we can measure everyone else, and if she -- the purest Puritan in the play -- can be convicted of witchcraft, then there can be no doubt whatsoever that the accusers are corrupt, as is the court that convicts her. Her purpose, then, is to show just how debased and degenerate the girls, their families, and the judges are.