To limit the probability that cracks in the masonry or gaps between the masonry and the liner will lead to the ingress of precipitation or melting snow into the chimney, which could lead to damage to the masonry or to the liner due to freeze-thaw stresses, which could lead to the leakage of flue gases into living space, which could lead to negative effects on the air quality of indoor spaces.
This is to limit the probability of harm to persons.