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.