My grandfather built this house = This house was built by my grandfather
Careless driving causes many accidents = Many accidents are caused by careless driving

We only mention the subject  when it is important to say who or what is responsible for something. When this happens we use BY to introduce the agent in the passsive sentence. For ex: in the next sentence the agent would not be mentioned as it doesn´t add any relevant  information .

Somebody cleans this class every day = This class is cleaned every day


It is used to talk about an instrument used by the agent to do something

the car was towed with a crane

 The room was filled with smoke
This cake is made
with brown sugar