What is the purpose of the prologue in Romeo and Juliet? Why is there a prologue?

Romeo and Juliet may seem to lack the complexity often associated with Shakespeare's other plays because of its apparent simplistic themes and flow and the foregone conclusion, as indicated in The Prologue to the first act. However, The Prologue ensures that the audience is not fooled by circumstances or appearances, immediately setting out that the lovers will "take their life" (6). The audience is then under no illusion as to the extent of the damage that an "ancient grudge" (3) can do and The Prologue also quickens the pace, revealing how fast matters can spiral out of control, almost unnoticed, until it is too late. It also reveals that Romeo and Juliet become the sacrifice, as if some greater good can be achieved by ending "their parents' strife" (8). The Prologue is also intended to point out that anything that is not clear from the Prologue will become increasingly clear as the play unfolds, which is what Shakespeare means when he says, "our toil shall strive to mend," (14).  

Staging a play with a predictable ending requires action and passion from beginning to end. The Prologue to the second act is the only real interruption to that flow. It gives the audience a chance to prepare itself for the foregone conclusion. However, this prologue also serves to build suspense and reveals how the clash between the "extremities" (Prologue.II.14), being the two warring families, actually fuels Romeo and Juliet's love and strengthens it so that "passion lends them power" (13). This adds irony reiterating and confirming that the "grudge" is responsible for the tragic end. 

Interestingly, there are no further prologues to the subsequent acts as the play requires no further explanation and the series of events completes the cycle. If any member of the audience has any doubts, he or she need only consider the Prologue to Act I and the confirmation of events in the Prologue to Act II to remove confusion. The audience could even get so involved in the events as to wish to shout out to Romeo at the end in order to save him from his fate. The audience is completely absorbed and their own powerlessness makes it even more dramatic.