Good question Zenith63. However, it is a safety procedure to ensure that the electrician and/or his mate does not get electrocuted and remains in control of the work. An electrician worth his/her salt would never trust anybody to leave the on/off master switch at the fuse board unengaged.
If you start your wiring from the fuse board you are putting your life in danger. All it takes is for somebody to switch the board on and the possibility increases of an accident. I know of a real case where an electrician was killed this way by the lady of the house anxious to put on the dinner who turned on the fuse board. Incidentally, he was wiring an outside shed.