To limit the probability that the ingress of precipitation or melting snow will lead to the premature failure of exterior finishes, structural wood elements or interior finishes, which could lead to the collapse of these building elements, which could lead to harm to persons.
To limit the probability that water from rain or melting snow will enter the chimney, which could lead to the structural failure of the masonry due to freeze-thaw stresses, which could lead to the collapse of the chimney, which could lead to harm to persons.