Here is how I explain it in my classes.
On the load side of the service there is a white wire and green wire. On the line side of the service, the white wire has an imaginary green stripe (since they are connected by the SBJ) If there is a line to case fault, the service neutral returns the current to the source (transformer), and back to the OCPD, which should open. With out a neutral, the line to case fault won't clear.