The nurse is more like a mother to Juliet than her actual mother. If you will look at the conversations that Juliet has with her nurse, they are much more loving than the conversations with her mother. Juliet calls her mother "Madam." Notice the terms of endearment that the nurse uses when she speaks to Juliet. She calls her "lamb", "my sweet" - things like that.
The nurse gives Juliet loving advice. They have an intimate relationship. Also, the nurse cracks many jokes and has some pretty racy conversations with Juliet. I believe Shakespeare used her character to add some light-heartedness to the play, which is very tragic. At times, she provides what is known as "comic relief."
Read the analysis here on eNotes.
In addition to providing necessary comic relief (for both the groundlings and those in the "expensive seats"), the nurse gives the audience/reader the opportunity to explore the classism and sexism extant during the time of the drama. Additionally, the nurse often exposes the relatively surprising powerlessness of the wealthy family she has served faithfully. She has not only been nurse and surrogate to Juliet, she has also provided the advice, security, and confidence normally provided by the child's biological and extended families.