To limit the probability of the premature failure of supply fans, which could lead to excessive negative pressure, which could lead to the spillage of carbon monoxide gas from fuel-burning appliances that are susceptible to spillage, which could lead to the acute poisoning or asphyxiation of persons.
To exempt supply fans from the requirement to be approved to handle untempered outdoor air, in situations where the outdoor makeup air is tempered before it reaches the supply fan.