The main advantage of the third-person point of view is that it allows Mason to highlight the growing sense of estrangement that Leroy feels from Norma Jean. Since his workplace accident, Leroy has noticed that his wife has become more distant, and the third-person point of view reflects that distance. To Leroy, it seems that Norma Jean resents his constant presence around the house, where he spends all day making handicrafts. After Leroy was forced to give up his career as a truck driver, Norma Jean suddenly found herself thrust into the role of sole breadwinner. She works in a drugstore to make ends meet.
Inevitably, this unexpected role reversal has created tension, and it's hardly surprising that Leroy feels—rightly or wrongly—that Norma Jean has less affection for him than she used to. The third-person point of view is ideal for conveying Leroy's feelings; Norma Jean's rapidly becoming something of a mystery not just to Leroy, but also to the reader.
The weakness of such a narrative approach is that it doesn't provide us with a truly objective view of Norma Jean. Because she's largely a mystery, she doesn't really appear to us as a three-dimensional character. As we're mainly given Leroy's side of the story, rather than hers, we're unable to develop much in the way of empathy toward her. We can certainly sympathize with Norma Jean over her plight: she's expected to toil away each day at a hard, low-paying job just to stay financially afloat. But at the same time, the third-person point of view puts us at a critical distance from Norma Jean, meaning that we can never really feel for her in the way that we can with Leroy.