The plot provides both a setting of decay and creepiness that lends itself well to the rather morbid and disturbed events of the story, as we discover them at the end. It also provides symbolism for Miss Emily herself, and for the state of her family and her life. The small town atmosphere also contributes well to the style of narration, which tells the story of Emily all through the collective voice of gossipy townspeople.
At its core, the story is about a deeply disturbed woman who goes to unnatural lengths to feel loved and accepted. Think of modern-day horror movies, whose setting always takes place in decrepit, shadowy, run-down mansions. Faulkner describes the house as such:
"only Miss Emily's house was left, lifting its stubborn and coquettish decay above the cotton wagons and the gasoline pumps-an eyesore
It's a classic creepy tale setting. Faulkner then makes the symbolic connection by saying that Miss Emily herself was like the house, a leftover from another time, decaying and no longer important as she and her family used to be. The fact that her family used to be big money, the "celebrities," elicits just enough curiosity for the townsfolk to follow her story and gossip about it, which allows for the story to be told through them.
I hope that these thoughts help to get you started; good luck!