To limit the probability that depressions in vent pipes will lead to an accumulation of liquids from condensation or backflow, which could lead to obstruction of the vent capacity, which could lead to inadequate venting, which could lead to siphonic action or back pressures in drainage systems, which could lead to trap seal failure, which could lead to the entry of sewer gases into occupied space, which could lead to negative effects on indoor air quality, which could lead to harm to persons.