What is the purpose of active and passive voice?

Question

In both passive and active voice, the meaning of the sentence is same.

So what is the purpose of passive and active as by two different names?

Answer 1

The sentence may recount the same facts, or it may not. One very significant difference is that in the active voice you must identify the Agent: the person (or other entity) who performs the action, whereas in the passive you are free to omit the Agent.

Active

^ok John stole my book.
^∗  Stole my book.

Passive

^ok My book was stolen by John.
^ok My book was stolen.

In either case, the passive voice is employed to focus on the Patient of the action—the person or thing which 'suffered' it—rather than on Agent who or which performed it. There are several reasons you might want to do this:

• Because you don’t know or don’t care who performed the action
• Because what is important is the action itself rather than who performed it
• Because what is important is the result of the action rather than who performed it
• Because you want your readers to think of the action as 'impersonal'

The last reason became particularly important in the 19th century, when scientific writers—or writers who wanted to be thought of as adopting a 'scientific' attitude—were particularly careful to banish any suggestion that the writer's personality or attitudes had any influence on what was reported. The passive voice remains to this day something of a fetish in the sciences, particularly in the 'soft' sciences, where there is still substantial anxiety about the scientific status and value of the work done.

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^∗  marks an utterance as ungrammatical.

Answer 2

I would like to invite us to consider what would happen if the passive voice did not exist.

To summary StoneyB's answer, the passive voice allows us to not have to mention the Agent (the person who performed the action). This is useful when the Agent is less important, unknown, or someone or something you don't want to mention.

What if we don't have the passive voice?

Consider this text from a Wikipedia page, Merchant of Venice:

Though classified as a comedy in the First Folio and sharing certain aspects with Shakespeare's other romantic comedies, the play is perhaps most remembered for its dramatic scenes, and is best known for Shylock and the famous "Hath not a Jew eyes?" speech.

Who classified it (the play)? Who remembers it? Who knows it?

It's unclear who did and does (or do) so. The passive voice allows this. Without the passive voice, we would be forced to state the subjects of those clauses explicitly. (And, who forced us so?) The most practical way to rephrase that sentence in active voice is to assume either "we" or "people" as the subject. Let's see what happened if I rewrite the sentence in active voice,

Though people classify the play as a comedy in the First Folio and the play shares certain aspects with Shakespeare's other romantic comedies, people perhaps remember the play most for its dramatic scenes, and know it best for Shylock and the famous "Hath not a Jew eyes?" speech.

And that is quite awkward.

Let's consider another sentence.
^{(the credit goes to Cerberus, who give me this great example sentence in our ELL chat room)}

When the dam collapsed, the water was released into the valley.

Who released the water?
The incident? The dam itself? The officers who are responsible for the dam?

And because I don't know who released it, I think we are lucky to have the passive voice around.