Our English teachers taught us to avoid the passive. They said it was weaker and more cumbersome than the more energetic, more compact active voice. That was bad advice. Very bad advice. Lawyers cannot write sophisticated, powerful prose without a skillful use of the passive voice. I could offer you a theological proof: God would not have created the passive had it no use. Or perhaps you might prefer the Darwinian argument: The passive could not have survived unless it was fittest for something. But I prefer this circular reasoning: The passive is better than the active in all cases in which the passive does a better job than the active. It only remains for a writer to recognize those cases and to know how to handle them.
Since grammar is often left untaught, I had better demonstrate the distinction between active and passive. In the active voice, the grammatical subject of the sentence is the agent (the doer) of the action: