Dramatic purpose is a term used to describe a scene or part of dialogue in literature that serves a specific purpose to the plot. There are several factors that can make a scene or speech have dramatic purpose.
For scenes, dramatic purpose may exist if the scene establishes setting or mood, reveals character traits or opposition, shows or creates relationships between characters, connects plot points, advances or complicates the plot, revels themes or motifs, or creates pathos.
For speeches or dialogue, dramatic purpose may exist if it advances the plot, explains or reveals a situation or character, establishes mood or atmosphere, provides tension relief or creates tension, reveals the theme or motif, or uses irony, foreshadowing or other literary devices.
Pieces like Romeo and Juliet are rife with dramatic purpose.
For instance, when Juliet refuses to marry Paris her mother says "I would the fool were married to her grave!" This reveals the nature of the relationship between Juliet and Lady Capulet, reveals in part the conflict between Juliet and her parents and the oppositional character to her love, Romeo. Additionally, this foreshadows the end of the play when Juliet will die for her love, her marriage and grave intertwined.
Even scenes such as the introduction of Romeo to Juliet have dramatic purpose. It launches the star-crossed lovers' conflict in the story, introduces the primary characters, and reveals their relationship. These are just a couple of the numerous examples of dramatic purpose in Romeo and Juliet.