The document discusses three reasons for using the passive voice: 1) When the subject is unknown, 2) When the subject is unimportant or we don't want to mention it, and 3) When we want to focus on the object rather than the doer of the action. Some examples are provided for each case, such as "His car has been stolen" when the subject is unknown, "A person has been arrested for murder" when the subject is unimportant, and "It is a very good film" when focusing on the object rather than the doer.