Он нужен, когда исполнитель действия (агент) неизвестен, неважен или очевиден, а фокус внимания — на действии. Если мы и упоминаем исполнителя, то через предлог by.
Посмотрите на примеры в пассивном залоге:
These cameras are made in China. — исполнитель неважен.I was born in 1986. — исполнитель очевиден (всех рожают мамы).His wallet was stolen. — исполнитель неизвестен.Структура построения пассивного залогаS + BE + Ved/V3S — это подлежащее. Главное, что следует помнить — в пассивном залоге ВСЕГДА есть глагол to be (который изменяется в зависимости от времени) и смысловой глагол всегда с окончанием -ed, если он правильный или в 3 форме (причастие если он неправильный.
Сравним предложения активного (действительного) залога с соответствующими предложениями пассивного (страдательного) залога в разных временах.
Active VoicePassive VoicePRESENT SIMPLE（происходит регулярно, «вообще»）S + am/is/are +Ved/V3I wash my car every week.
It is quite big with sport ground behind it, inside swimming pool.
On the groundfloor there are the classrooms for the primary-
school pupils, workshops, library.
There are all kinds of tools and machines in the workshops.
The boys of our school have a woodwork room too.
There is a room for manual works for girls.
Teachers teach them how to cook, sew and design clothes.
Our school library is nice and clean.
Two librarians help pupils to find books they need.
There are many bookcases and bookshelves with a lot of books there.
If you enter the school and turn right you see a big light dining-room.
It is always busy and noisy, but it is clean.
Here pupils and their teachers have their lunch.
There are blue curtains on the windows and beautiful pictures on the walls.
There is a gymnasium on the ground floor as well.
Our physical training lessons are held there.
Pupils like to go there even after the lessons, because it has a lot of sport equipment.
Our school has many classrooms.
The classrooms are light and spacious.
There are three large windows in each classroom with flower pots on the window sills.
Each room has teacher's table, pupils desks, blackboard, tables and charts on the wall, maps and portraits.
There are special classrooms for Chemistry, Physics, Biology,
History, Geography, English, and Russian.
On the third floor there is a big nice assemble hall.
A lot of meetings, concerts, festivals are held there.
Our classroom is on the second floor. Its windows face the school-yard.
Our form-mistress is a teacher of Russian language and literature.