Huston's use of dialect is important in helping us to understand and these characters and the novel as a whole. Because so much of the novel is concerned with Janie "finding her voice," it is essential that as readers we hear that voice in our minds, that we too "find her voice."
Hurston's background as a folklorist and anthropologist is also critically important in this regard. Not only is Hurston capturing the experiences of her main character, she is also weaving in historical information regarding her hometown of Eatonville and much of the life and folklore of the emerging African American culture.
In short, without the dialects the novel would not hold the power that it does. Part of "magic" that makes this story work so well is the authenticity with which it is told.