To limit the probability that the performance of cement will fall significantly below expectations, which could lead to the stucco being of inadequate strength, which could lead to delamination or cracking under gravity, wind or impact loads, or freeze-thaw stresses.
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.