Our main supply duct has motorized fire damper .why do we need branch duct FD need to be motorized due to fire suppression system?

SUPPLY AIR DUCT DAMPER AND RETURN AIR DUCT DAMER ARE CONNECTED IN REVERSE MANNER WHEN ONE WILL CLOSE THEN ANOTHER WILL OPEN BECAUSE IN CASE OF FIRE SUPPLY AIR WILL CLOSE DUE TO PREVENT FIRE SPREAD OVER A FLOOR ANOTHER ONE (FD) WILL OPEN FOR FESH AIR FOR PEOPLE