<<But more importantly - why board with no pilots.>>
Many times, flight duty limitations, at least in our case.
At a remote parking bay, pax boarding is normally a long process (362 pax, in our case) and the FD flight duty periods are normally long (16 hours max) but CC are allowed to go to 18 hours, same for ground engineers (we carry a minimum of two, always) so it makes perfect sense.
Fire on the ground?
Ground engineers are always present, and they have been properly trained for such occasions....as are CC for cabin difficulties.