The fire doors in buildings are there to protect your life and property. These doors keep a fire that's in the hallway or on another floor from blocking your escape or getting inside your apartment.
Because building stairways and hallways can act like chimneys, a fire travels quickly from one area to another until it is slowed down by a closed fire door.
The next time you find a fire door on your floor propped open, close it. The life you save may be your own. It's against the law to prop open a fire door.