Passive sentences are all about changing the focus of what you are talking about.
For example: Somebody stole my bike (this is a normal ACTIVE sentence)
Subject, verb, object
Subject = Somebody (the person doing the verb)
Verb = stole (the past simple of 'to steal')
Object = my bike
But I am not interested in that horrible 'somebody'. I am MORE interested in my poor little bike.
So, by changing to a sentence with passive grammar, the whole focus of what I am saying changes.