To limit the probability of excessive deviation from the optimum mortar-joint thickness required to ensure continuous support of masonry units while maintaining adequate mortar-to-masonry-unit bond strength and adequate compressive strength of assemblies.
This is to limit the probability of precipitation or meltwater ingress, which could lead to deterioration due to freeze-thaw stresses.
Where masonry construction is required to provide fire resistance, this is to limit the probability of compromised fire resistance of the assembly, which could lead to harm to persons.