why does the play begin with a speech from the Nurse? what i the dramatic purpose of her speech ?

Euripides has the opening of the play delivered by two slaves, a Nurse and a Tutor. An important feature of his work is allowing slaves to speak, and speak well. The Nurse and the Tutor provide their perspective on the events in the house they serve. Significantly, both of them condemn Jason. The Nurse, after a few brief moments on stage, is already well-defined as a character. She is loyal to the house and to Medea, but she fears Medea and her violent heart. There are differences of attitude between the two slaves, and these differences seem to break down along the lines of gender: the Nurse seems to be shocked by Jason's behavior, while the Tutor cynically remarks that everyone looks out for himself. The slaves provide an outsider's eye on the action, and they are canny enough to predict events. The Nurse's fears foreshadow the terrible fate of Medea's children. And yet the slaves are completely powerless to alter the course of events.

Passion is an important theme of the play. The Nurse reminds us that Medea is here because she followed Jason back to Greece out of love.