What is the purpose of the backwards movie episode in chapter four?
The entire novel deconstructs linear time, and watching the movie backwards is an enactment of this. Watching the movie backwards triggers in his mind memories of WWII, here imagining it going back to the beginning of time. This imaginative flight signifies the timelessness of war and perhaps, too, the repeated evidence that humankind is violent and cruel. Throughout the book Billy goes back in time or out of time, so to watch a movie backwards makes "sense" in this sort of logic.
Even though the pictures in the movie are physically the same when played backwards and forwards, the narratives we attach to these pictures are different.
When played forwards, we decry the bombing.
When played forwards, we celebrate the return of things to the earth.
The order matters. It chages the value that we attach to the pictures.
Also, watching a war movie backwards changes the "story" of the movie, which would be a story of destruction, if watched in normal chronological order. As it is shown backwards, the movie shows chaos to creation. Rather than everything being destroyed, killed, and pulled apart, it is made whole and born anew.