When Do We Use The Passive Voice?

The passive voice is used (spot the deliberate example!) to give you more flexibility over how, or whether, you talk about the agent (person or thing doing the action) in a sentence. Let's say you don't  know who does an action, or it's not important. There's no point saying "They make wine in France" or "Someone will meet you at the airport" when what matters is the action itself, not who does it. So you use a passive sentence: "Wine is made in France, "You will be met at the airport."
Alternatively you may wish to highlight , rather than ignore, the agent. The passive is good for this too: "Hamlet was written by Shakespeare" makes Shakespeare stand out more than the active "Shakespeare wrote Hamlet."
The other main use of this form is to express a general idea. "It is believed that the prisoner is now in France" or "The prisoner is now believed to be in France" is more economical than "Many people believe that..."