In Romeo and Juliet, Mercutio is there at the beginning of the play and tells Romeo to forget abut the girl he loves, Rosaline. He tries to get Romeo to find a different girl and so they go to a masquerade ball and he meets Juliet, his new love.
I would say to develop character and to make the audience think it's funny, most likely.
I hope that helps!
- mathwizzard3