I'm going to assume you mean Act III, scene 3, where the Friar launches a tirade against Romeo and his seeming cowardice. He compare Romeo to a beast and a woman, bringing a certain power to his speech, while at the same time suggesting Romeo's unbelievable irrationality. For Elizabethan audiences, women and animals were considered equally devoid of logical and reason, so the Friar's remarks here would resonate with those watching.
The Friar's apostrophes also serve to chastise Romeo for his behavior. At different points in this scene, the Friar laments "O deadly sin! O rude unthankfulness!" and "O woeful sympathy! Piteous predicament!" It's as though Romeo's reaction has driven him to crying out, identifying Romeo's faults as he does so. He also discusses Romeo's shape love and wit, saying each is destroyed or perverted by his behavior. By addressing each of these aspects by name, the Friar is logically breaking down Romeo's illogical response.
Finally, his rhetorical questions serve the same purpose. As he asks Romeo each question, he forces Romeo to think through his decisions and his desires, pointing out how harmful each can be:
Hast thou slain Tybalt? Wilt thou slay thyself?
And slay thy lady that in thy life lives,
By doing damned hate upon thyself?
Why railest thou on thy birth, the heaven, and earth?
Each of these devices serves to drive home the Friar's point to Romeo, which is: Wake up and consider yourself lucky!