1. When we want to change the focus of the sentence:
The Mona Lisa was painted by Leonardo Da Vinci.
2. When who or what causes the action is unknown, unimportant, obvious or
'people in general':
He was arrested . (obvious)
My bike has been stolen . (unknown)
The road is being repaired. (unimportant)
The form can be obtained from the post office. (people in general)