To limit the probability of an inadequate width of cavities, which could lead to extruded mortar bridging the cavity, which could lead to water crossing the cavity and penetrating the interior wythes by capillary action, which could lead to the deterioration of the interior wythes.
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.