Why would an author use an unreliable narrator?

Because unreliable narrators are telling a story that isn't quite, accurate or credible. Do to their mental state/maturity.

Answer and Explanation: An unreliable narrator could be used by the writer to create a text with different levels of truth. By using an unreliable narrator the writer might give the readers the possibility to use their own interpretation of the events to reveal what is reliable and what is not.