To limit the probability of water leaking through joints in precast concrete or masonry chimney caps, which could lead to water penetrating the masonry or entering between the masonry and the liner, which could lead to damage to the masonry or liner due to freeze-thaw stresses, which could lead to the spillage of combustion products 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.