To limit the probability of incomplete coverage of aggregate with cement paste, which could lead to stucco being of inadequate strength to resist gravity, lateral, impact or abrasion loads, or freeze-thaw stresses, which could lead to cracking.
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 escape of heat or flames, which could lead to the ignition of combustible building components, which could lead to harm to persons.