There are two goals in IV.ii:
Comic Relief: Shakespeare likes giving his audience a break, and he typically uses his low-class or peasant characters to deliver some sort of comedic break, especially during his tragedies. In the first half of IV.ii, Capulet's interactions with the servant are meant to lighten the mood, especially given the preceding scene's gravity.
- Dramatic Irony: Dramatic irony is a literary device when the audience is aware of something that the characters are not. As a result, the audience perceives certain statements or actions totally differently from the characters in the play. In the preceding scene, Juliet and the Friar have determined a way for her to fake her death and thus escape her parents' desire to have her marry Paris. However, in IV.ii, Juliet appears to acquiesce to her parents, much to their delight. Unfortunately, the audience realizes that Juliet is just trying to make her parents happy before she makes them tragically sad.
The above answer is excellent! Shakespeare intended to take a short break from the intense action that the audience/reader was experiencing and release a little tension through laughter. It gives the audience a chance to refocus, and (although they don't know it yet) prepare for the remaining plot points to come. This short scene will be the last moment of any levity for the audience or the characters.