To provide a smaller earth resistance path, so a greater current will flow in the event of a fault, this will reduce the trip time of the circuit breaker, or earth leakage device.
Or as we say "the bigger the bang, the better the job".
I am assuming the resistance is quite different because the neutral conductor has a much smaller resistance due to it being much thicker than the outer sheath in an SNE cable?
Yes bigger cross section lowers the resistance, in a SNE, they use 3 conductors, in a CNE (PME in Europe) they use 2 conductors, the Neutral and Earth are one conductor, which is separated at the incoming fuse board by a copper bus bar.