When is it an appropriate choice in writing to use the passive voice

When the subject (person or thing) performing the action is not relevant.

Explanation:

The Passive Voice when writing or speaking is used when you wish to focus the attention on the person or thing affected by the action; instead of focusing on the person or thing who/that performs the action. I hereby give you an example.  

As you can see, the second option does not include a subject/pronoun/person, and even if it were, the most remarkable there would be the letters and not the person who performed the action, whereas in the first example, it is remarkable the fact that “they” sent the letters; not you, not me. You can as well, include a name or be more specific:

you have to tell us what you're talking about or it's impossible to answer.