What is their purpose?

Fire doors create a passive fire safety system, protecting commercial buildings from the spread of fire by confining it to a specific area for a longer period of time. This allows people within the building enough time to escape via alternative routes which might otherwise be cut off by fire.

They are a legal requirement for particular buildings, including for flats and commercial property, but the prime purpose for fire doors is to save lives and halt the progress of a destructive fire.