In Rebecca, Mrs. Danvers is in some regards the voice of the missing Rebecca. Danvers adored and continues to adore Rebecca and feels that the new Mrs. de Winter is a usurper who has no legitimate right to a place at Manderley. Therefore, Mrs. Danvers is significant throughout Rebecca in that she keeps Rebecca alive and a palpable presence. She might also be considered symbolic of the true nature of Rebecca's character: if one so patently evil ("The face of an exulting devil") could so worship Rebecca, then the suggestion is early on planted that Rebecca must have been in truth horrible.
Mrs. Danvers is also important to the character development of Mrs. de Winter and to the development of the relationship--bred of misunderstandings--between Maxim and his new wife. Danvers terrorizes Mrs. de Winter while appearing to play the part of a loyal and helpful servant to Maxim, all the while hiding from him her adoration of Rebecca while parading it before Mrs. de Winter. Therefore, Mrs. Danvers is significant throughout because she drives the dramatic development of the primary characters and the suspense of the novel.
Without Mrs. Danvers, there would be no mystery to this novel because it is she who keeps Rebecca's ghost alive. We learn the truth about Rebecca and how evil she was by the end of the novel and we also learn that Maxim came to despise her, and that he was the one that killed her. So, Rebecca's memory would not even come into play if it had not been for Mrs. Danvers. If not for Mrs. Danvers, the new Mrs. DeWinter would not be so insecure, would not think that her husband is still in love with Rebecca. Mrs. Danvers moves the gothic nature of the novel forward by trying to get rid of Mrs. DeWinter. She dresses her in Rebecca's clothes, she tries to convince her to commit suicide, etc. If it were not for her, Maxim would have brought his new wife home and they would have learned to build their life together, in spite of his past tragedy with his first wife, Rebecca. In Mrs. Danvers, we have a living evil, whereas Rebecca's evil is part of the past. Mrs. Danvers keeps Rebecca's memory and her ghost alive.