To limit the probability that, on severe overheating [i.e. during a chimney fire], the expansion of the liners will put stress on the masonry surrounding the flue liners, which could lead to the failure of surrounding masonry or the displacement of liner sections, 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.