To limit the probability that the base coat will extract and absorb water from a relatively thin finish coat of stucco, which could lead to negative effects on curing of the finish coat, which could lead to an inadequate resistance to abrasion or freeze-thaw stresses, which could lead to delamination.
Where stucco is applied to masonry chimneys, this is to limit the probability of precipitation or meltwater ingress, which could lead to the premature failure of chimney liners, which could lead to the leakage of carbon monoxide gas into living space, which could lead to the acute poisoning or asphyxiation of persons.