Often a writer needs to steer the reader’s attention away from the agent of an action. The passive allows him to do so because the agent can be left unmentioned, which is impossible in the active voice. You can say Pooh ate the honey (active voice, actor mentioned), The honey was eaten by Pooh (passive voice, actor mentioned), or The honey was eaten (passive voice, actor unmentioned)—but not Ate the honey (active voice, actor unmentioned).
