Sula is told in the third person, and the narrator is able to let us in on the inner thoughts of nearly every character in the novel. Since the story is so character-driven, the third person omniscient narrator grants us important access to the contradictory figures who propel the action. We get to know Helene as much as we do Hannah, Sula as much as we do Nel, Shadrack as much as Eva. This is especially useful in helping readers reserve judgment, since the narrator doesn't seem to judge them either.
The third person narrator also allows us to understand certain things about characters without them having to tell us or each other. For example, Sula never tells anyone that she chops off her finger in an attempt to be like Nel; we only learn this because we have access to her thoughts when she's on her deathbed. Much goes unsaid in the novel, especially between characters, but the third-person narrator clues us in to their thoughts and motivations.