Why does Shakespeare use so much foreshadowing throughout Romeo and Juliet? How does the reader benefit knowing that they will die?

Shakespeare's use of foreshadowing to let readers know that Romeo and Juliet are destined, as "star-crossed lovers," to fall in love and die creates dramatic irony, increasing suspense and tension for the audience, and ultimately making the emotional catharsis upon the play's resolution that much more fulfilling.  Dramatic irony occurs when the audience knows more than the characters, and its main function is to increase tension. 

Consider the end of the play when Romeo is in Juliet's tomb, about to drink poison because he does not know that she is really alive.  However, we, the audience, do know.  This moment is so stressful for us because we know that Juliet will wake up any minute, and if Romeo would just delay a moment or talk a little longer, then she would awaken and they could be together.  Then Romeo dies, and we see Juliet wake up, not knowing that her husband is dead.  It is such a near-miss, and we see it all unfolding, knowing how it must, and this quintessential example of dramatic irony shows how this device increases audience tension, heightening the tragedy.  This also intensifies the emotional catharsis (the purging of the tension created by the irony) we feel when everything is finally revealed to the other characters.  Though the Capulets and the Montagues only just now come to understand that it was their hate that ended their children's lives, the audience has understood this from the beginning.  It is not a surprise, then, to us, which allows us to appreciate fully the surprise it is to them.