To limit the probability of an inadequate physical bond between the finish coat of stucco and its substrate, which could lead to an inadequate resistance to 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 escape of heat or flames, which could lead to the ignition of combustible building components, which could lead to harm to persons.