The active voice is most effective when the subject or "actor" is the focus. The active voice typically results in a stronger, more direct, and more concise statement. "John hit the pedestrian with his car and injured her."
The passive voice is preferred when the action or the object (recipient of the action) is more important. "The pedestrian was injured badly when John hit her with his car."
Passive voice is also used when there is an attempt to distance the subject or actor from the action. "The cigarette ads were designed specifically to target children." vs "We designed the cigarette ads to target children."