Faulkner zigzags all over the place, from one temporal point to another, but for essential reasons. What are those reasons?From one temporal point to another, why?
The first reason Faulkner veers back and forth in time can be seen in the early descriptions of the house in its community:
"It was a big, squarish frame house that had once been white, decorated with cupolas and spires and scrolled balconies in the heavily lightsome style of the seventies, set on what had once been our most select street. But garages and cotton gins had encroached and obliterated even the august names of that neighborhood; only Miss Emily's house was left, lifting its stubborn and coquettish decay above the cotton wagons and the gasoline pumps-an eyesore among eyesores."
This entire community is one where the past and the present are interwoven. The past can't be forgotten or abandoned, and the present presses on the past (and vice versa).
After that, look at her defense for not having to pay taxes: the past presses on the present as well.
A third reason is that the story contains a mystery. In mysteries, after a death, the events leading up to the crime are reconstructed, and a meaning is sought in the sequence. That happens here.
I agree with everything in the first answer but I'd like to add that the story itself is told from the memory of a townsperson. Memory is not always chronological, but can flit from one moment to another without regard for time. Thus, Faulkner is also imitating memory when he zigzags in time. He uses this same technique in many of his stories and the challenge for the reader is to understand how, for Faulkner, the rules of time do not allows apply to his thoughts.