In some ways, the marriage of Romeo and Juliet can be understood as a plan to reconcile the two squabbling families in Verona. This, at least, is why he hastily agrees to Romeo's request that he officiate the ceremony. Even as he admonishes Romeo for his impetuousness, he makes a decision that, in the end, makes things worse.
But the Friar's other plan, one on which the entire plot turns, is for Juliet to consume a potion that will put her into a deep sleep that will convince her family that she is dead. In this way, she can avoid marrying Paris, as her father has decreed, and, after Romeo has been alerted to the situation, leave for Mantua with her husband. In the meantime, her family, believing her dead, will place her in the Capulet family crypt.
Of course, the plan goes awry, as, due to plague, Friar Laurence's message does not reach Romeo. Romeo's friend Balthasar does, however, and he tells the exiled Romeo that he has witnessed Juliet's funeral. Believing that his wife is actually departed, Romeo goes to the crypt, where he commits suicide (with authentic poison) next to Juliet's lifeless body after killing Paris in a duel. Juliet duly awakens to find Romeo's body, and grief-stricken, she too kills herself. Thus Friar Laurence's plan backfires, and the two lovers meet their tragic fate. On the other hand, the death of Romeo and Juliet accomplished what their marriage could not. Capulet and Montague are reconciled at the end of the play.