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.
Rebecca Nurse, like John Proctor, is there to stand for the voice of reason. She is older, respected, and has served as a midwife for many years. She is logical, and has a no nonsense way about her. She and John are among the first to blow off the idea of witchcraft among the girls. She is called in to see Betty and Ruth, and her opinion is that when the girls are ready, they will wake up. There is nothing wrong with them physically. She has much experience with this as she, herself, has many children and grandchildren. None of her children or grandchildren have ever died in birth or in early childhood which is unusual during the Puritan times. This is the reason why she is targeted as a witch...Mrs. Putnam's babies have all died except for Ruth. She accuses Rebecca Nurse of being a witch and killing her children simply because Mrs. Putnam is grief-stricken and jealous of Mrs. Nurse's fortunate state.
The fact that respected and well-thought-of members in the community of Salem are hanged like Rebecca Nurse, John Proctor, and others underlines the effects of social hysteria and fear. In any other situation, those people would have been the ones to calm the masses...not hanged by the very people who looked to them for guidance.