From the beginning of the play, Rebecca Nurse has served, along with John Proctor, as a voice of reason. She sees the children's deception for what it is, though interprets it as a "harmless" thing that children always do. Along with her insight into mischief comes a certain innocence as to the depravity to which the girls will sink. Well-respected in the community, she is not seriously suspect of witchcraft until the madness has fully taken over. She serves to heighten the ridiculousness of the charges.
Rebecca also serves as a kind of foil for John Proctor. Though both stand apart from the madness and the over-religiosity of Salem, Rebecca chooses to function within it, while John has removed himself as far as safely possible. It is in this role that she appears at John's signing of his confession.
Rebecca assumes, seeing John alive, that he has remained to the truth, as she has done herself. She is shocked to find he is about to sign a false confession. She thus serves to jolt John out of his mode of self-preservation and back into being a stalwart of truth. She reminds him of the higher truth of life, as opposed to the false interpretation of that truth that has plagued the community during the trials. Together, therefore, she and John Proctor lose the earthly battle, but gain the moral victory by dying for the truth.
Act 4 begins with the town in total disarray. Orphans and cattle roam the streets; fields are left unplowed, and people are afraid to go out. Abigail has absconded with her uncle's money, and the judges are forced to supply some validity to their decisions. Rev. Hale is now completely against the court, and even Rev. Parris is working to reverse the judges' sentences (if only for selfish reasons, this is still a complete turnaround for him). At this point in the play, two of the town's stalwart leaders Rebecca Nurse and Martha Corey are still imprisoned and sentenced to hang. The judges know that they will never be able to convince Rebecca or Martha to confess; so they focus on John Proctor. Miller includes Rebecca in this act as one of the victims who will die with John for a couple of reasons.
First, in Miller's previous description of Rebecca's words and actions, it is credible to the reader that the judges would not even consider asking her to confess to save their own reputations. Rebecca has never swayed from her beliefs and will not blacken her soul for their account. Thus, this sets up the scene nicely for the conversation between John and Elizabeth and the confession decision.
Secondly, John's sense of guilt causes him great conflict. He does not feel worthy to die in the company of Rebecca Nurse. He knows that she will not confess because she believes it to be wrong. Up until his signed confession, John has not confessed because he is proud and will not give the judges and Parris the benefit of seeing him break that pride. When John does decide to tear up his confession and go to his death, he demonstrates that he has forgiven himself for his adultery and now feels that he can stand with Rebecca and do what is right as an example to the town. He, like Rebecca, can keep his good name.
Thus, Rebecca mainly serves as motivation for the tragic hero's final decision.