These doors are used at stairwells, between an office and corridor, and sometimes to separate different kinds of rooms, such as classrooms from laboratories. Fire doors have a self-closing device. They are tested by reputable laboratories to withstand fire for a certain length of time, typically from 20 minutes to 3 hours. If a fire door is left open, its safety protection is lost.
Fire takes the path of least resistance.
If a fire starts in a single office, and the door to the corridor is left open, the fire will spill out into the corridor while the building occupants are evacuating. If a fire door to the stairwell is left open, the smoke, and eventually the flames, can contaminate the very place that people seek for a safe exit. You may recall the story about the home of the mother of actor Macauley Culkins in Manhattan on the nineteenth floor on December 28, 1998. There were four deaths. An electric heater caused the fire, which spread because the doors onto the corridor were propped open.