To limit the probability that delays or ineffectiveness in shutting off the ventilation system will result in continued air movement in the ventilation system, which could lead to the spread of a fire in the power-ventilated enclosure to other parts of the building, which could lead to damage to the building.
To limit the probability of delays or ineffectiveness in turning on the ventilation system in the event of the accidental release of ignitable vapours or particles, which could lead to a fire or explosion, which could lead to damage to the building.