The interlock is required for hydraulic elevators with automatic recall. When the power goes out the elevator automatically returns to the first floor and lets the passengers out. When working on the system and you turn off the disconnect for the sake of safety you don't want the elevetor moving down to the first floor. When the NC mechanical interlock opens it tells the elevator not to recall to the first floor even though the power is out. It has been a while since I have done an elevator but I am pretty sure thats the reason behind it.
