咩時候要用passive voice?

想到以下情况會用passive voice:
(1) 告示，規則，通告，文字上的警告語
e.g.
Dogs are not allowed here.
No smoking is allowed in this room.
You are requested to bring you lettter of recommendation in the interview.
(2) 新聞標題
e.g. The house was burned down last night without any injury reported.
多數用passive voice前會唔會係物件多?
是。但亦可以是Pronoun如You, I , It 等