In "A Rose for Emily," the seemingly random revelations of the townspeople/narrator enable the narrator to shock readers with a surprise ending.
Placed chronologically, Emily's poisoning of Homer is painfully obvious. Relating the events to the reader in a random manner enables the narrator to surprise the reader with Homer's skeleton and the grey hair on the pillow. That is the direct answer to your question. Of course, the events only appear to be given randomly. The plot is obviously carefully manufactured.
The narration is the "we" of the town. The townspeople relate events from outside of Emily's home. The narration tells only what could be seen from the outside, not from the inside. Bits and pieces of Emily's history and actions--such as the buying of poison--are related to the reader as the person involved might have related it to others. Again, the pharmacist is an example.
The narration makes the surprise ending possible. The "we" relate only what they know at the time events occur during all of the flashbacks that dominate the story.
There are several stylistic devices that Faulkner chooses for A Rose for Emily, and given the fact that the point of view is a third person omniscient and is intended to be told from the perspective of the townsfolk, it would be much different to narrate the way a detached omniscient narrator would. Should a detached objective and omniscient third person narrator be telling the story, its chronology would be organized, sequential, and causative. However, the disruption in chronology is necessary for us, as witnesses and as "guests" in the story, to acquire the information at the same rate, and in the same pattern that the towns folk got it. That way we are all at tandem and we are all part of the action. Plus, the story is also told as gossip, and you know that gossip never comes in an organized nor sequential way either.