Why does Hurston devote this chapter to Janie’s jealousy of Nunkie?

Chapter 15 of Their Eyes Were Watching God features the story of Janie's jealousy of Nunkie. At this point in Hurston's novel, Janie and Tea Cake are working in the Everglades (the muck), and Nunkie is also working there. Janie thinks Tea Cake and Nunkie are flirting with each other and sees them in some kind of physical act that she imagines is sexual or about to become so. Janie and Tea Cake fight about his relationship with Nunkie (or, as Tea Cake argues, the lack of a relationship), but then their anger turns into passion. Tea Cake reassures Janie that he loves her and insists that Nunkie means nothing to him; Nunkie could never measure up to Janie.
Hurston may choose to incorporate the chapter about Nunkie for a couple of reasons. For one, it shows the passion Janie and Tea Cake feel for one another. Their relationship is unlike Janie's other two marriages. When she was married to Logan, it was simply for financial security, and there was no love or desire on her part. Her marriage to Joe Starks began as a playful flirtation but became a one-sided power play when Joe became mayor of Eatonville; he began treating Janie as a possession and separated her from the townspeople, breeding resentment in Janie. With Tea Cake, Janie feels that she can be carefree and playful again. She feels that he treats her as an equal (financially, she is actually his superior, due to Joe's inheritance); for example, he plays checkers with her. They go to the muck together and work in the Everglades. Janie's reaction to Nunkie indicates her feelings for Tea Cake in the sense that her jealousy means she is afraid to lose him. This was not the case with her other husbands.
A second possible reason for the Nunkie chapter is that it foreshadows some darker moments in Janie and Tea Cake's relationship. Although the marriage is much better than her previous two, it is not perfect. A couple of chapters after this, Tea Cake beats Janie to show his dominance over her. Later, he contracts rabies from a dog during the hurricane (he was trying to save Janie), and he goes insane. He tries to kill Janie, and she feels she must kill him. Their relationship ends up being violent and tragic, and the Nunkie episode is one of the early signs of the potential volatility in their marriage. Once she is cleared of responsibility in Tea Cake's death, Janie moves back to Eatonville and lives her life for herself.