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:

tom comes from a southern part of the hemisphere and down in the south they talk with a lot of slang the family doesnt quiet understand what tom is tryna say becaus ehe talks like a three year old but near the end they finally try and desihper toms letter and its a success i have no idea hwta book this is or how to even answere this question honeslty but i am a really fast typer so i did this actually incrediably quick with the swiftness of the flash