One of the most critical elements of the Prologue is that it represents the essence of exposition. The prologue provides the background needed to fully understand the context of the play and the dramatic situation that confronts Romeo and Juliet. The notion of the two families possessing antagonisms is introduced to us in the prologue and allows us to hold a greater sense of understanding about the situation in which both lovers are immersed. Told in a very lyrical and poetic sensibility helps to add to the idea that the depiction that is about to be absorbed is art and the prologue helps to introduce this to the reader.
In Shakespeare's Julius Caesar, Shakespeare continues a Greek technique and tradition by using a chorus in his play. A chorus traditionally supplies commentary on the action of the play, interpreting the play from the perspective of conventional wisdom or common sense.
Shakespeare's Chorus does this in Romeo and Juliet. The Chorus gives "The Prologue," and introduces the opening conflict in the play--the "ancient grudge," or feud between the Montagues and Capulets.
The Chorus also briefly outlines what the audience will witness in "...the two hours' traffic of our stage." The "star-crossed" lovers and their "death-marked" love are introduced, as is the loss of their lives. This establishes the tragic mood of the play, as the doomed lovers meet and fall in love and are prohibited from being together.
I think the Prologue in the play "Romeo and Juliet" by William Shakespeare adds a sobering note to the action for Elizabethan audiences right even before the play begins - just in case they were in any doubt that this might be a bit of romantic 'light entertainment." It tells us that these "star-cross'd lovers" are not going to have an easy ride - and why (all about the ancient feud between the tow families of Montague and Capulet, a fight where the beginning is lost in the mists of time.) It establishes a judicious tone in advance of the role of the Prince and foreshadowing the sorrowful speeches at the end, and the 'blind-eye' to the violence - when all is too little too late.
In my opinion, the Prologue does not add all that much. I think that it mainly serves as an introduction, the way that the trailer for a movie acts these days.
I think, though, that it has a little more of a role than that. It also gives us some basic information about the play. It tells us about the hatred between the Capulets and the Montagues.
Finally, I think the Prologue adds to the sense of impending doom. We know right away that the play is going to end badly. This allows us to see things in a much different way than we would if we didn't know it was going to end tragically.