what is the dramatic purpose of having the Nurse hurrying in and out of the room

The Nurse's first appearance Act 3 reinforces the shift to tragedy. Her inability (or refusal) to expediently share her news with Juilet echoes the earlier scene (II.iv), when she teased Juliet. However, whereas that scene was played for comedy, the same device becomes infuriating and cruel under the tragic circumstances. These parallel scenes establish the tonal shift of the play. As a side note, the parallel also reveals the complexities of the Nurse’s character. Though Shakespeare could have written her as simply a functional character, he instead gives her layers - she is defined by her service to a young woman whom she also resents.

In Scene 5, the nurse hurries in and out of the room knowing Juliet is making plans to escape her marriage to Paris. Because of this the Nurse attempts to change Juliet's course and speaks of Paris as a virtuous man worthy of her hand (thus revealing her underlying resentment of her young charge). In response to the Nurse’s patronizing description of Paris, Juliet shouts, "Ancient damnation!" (3.5.235). This serves as both reference to the Nurse's age and to the problems she must deal with, all of which have been created by a feud that has its roots in the older generation.