It was something NSE specified on both 159s and 442s from new. The door is for exclusive use of the driver to avoid passengers falling out - remember that these units spend a lot of time over 3rd rail (far more then other sprinters) and for most of that it's 4 track (with the 159s using the central tracks)
Just to elaborate, it's in order to ensure that the vestibule is empty if and when the Driver needs to open the doors to leave the unit, for example to use the signal telephone. It's a bit of an odd one to be perfectly honest, on any other similar unit the Driver would simply check the vestibule first, but I suppose NSE were being extra cautious. The Guard can open the doors using a carriage key, and the door control panel within the vestibule will function in the normal way.
FGW use a hired-in SWT 158, and when it's on a busy service (it used to run to Brighton, for example) the locked out doors are a considerable nuisance, causing big problems with the flow of pax trying to board and alight all at one end of the front coach. Many Guards would just open the vestibule doors up and lock them out, in which case I believe pax can then open the external doors from the inside (the buttons operate internally I understand in case anybody should be trapped in the vestibule).