Here are some reasons for using passive voice:
*The passive voice can change the emphasis of a sentence.
Jack won the prize. (focus on Jack)
The prize was won by Jack. (focus on the prize)
*Using the passive is a way of avoiding the naming of a specific person who is responsible for an action.
It has been decided to reduce all salaries by 10%.
*In descriptions processes, there is emphasis on the actions performed rather than on the people who performed them.
Then the packets are packed into boxes of twenty four.
*The passive is often used with say, believe, understand, know and similar verbs used in reporting to avoid an impersonal they or people.
John Wilson is said to live in New York.
John Wilson is said to be travelling in Africa.
John Wilson is said to have arrived in Australia.
Hope that helps.