When writing a short story, what is the purpose of a story map, or plot diagram?

Read the excerpt from "the love song of j. alfred prufrock." let us go, through certain half-deserted streets, the muttering retreats of restless nights in one-night cheap hotels and sawdust restaurants with oyster-shells: how is this excerpt an example of dramatic monologue? the narrator is using long, interconnected sentences. the narrator is able to see into two character's heads. the narrator is addressing the audience directly. the narrator is jumping from one topic to the next topic