Although Mr Utterson's telling of the story is technically the first person point of view, he is nevertheless an outsider looking upon a situation he doesn't understand. By witholding critical information - that Dr Jekyll and Mr Hyde are indeed one and the same person - Stevenson creates both intrigue and suspense. The reader deduces the truth about the same time as the narrator and his friend Lanyon and shares their horror upon learning the truth. Note that Mr Utterson, being a lawyer by trade, would try to take an unbiased position despite his friendship with Dr Jekyll. Lanyon, more subjective by nature, trusts his "gut feelings" and is quicker to come to the truth. Both would intervene to help if they could, despite the atrocities Hyde commits. This heightens the conflict of the story (and therefore its literary interest).
Stevenson could have written the story from Dr Jekyll's point of view. In this case it would have been a type of confessional narrative, such as Poe's "The Black Cat," "The Tell-Tale Heart," or "The Cask of the Amantillado."