To limit the probability that the normal operating exhaust capacity of the principal ventilation fan will exceed the outdoor air supply flow, which could lead to depressurization of the dwelling unit, which could lead to the spillage of combustion products from fuel-burning appliances that are susceptible to spillage.
This is to limit the probability of the entry of carbon monoxide gas into living space, which could lead to the acute poisoning or asphyxiation of persons.