To limit the probability that extruded mortar will bridge the gap between the veneer and the sheathing membrane, which could lead to the accumulation of water on the sheathing membrane.
Where masonry construction is required to provide fire resistance, this is to limit the probability of precipitation or meltwater ingress, which could lead to deterioration due to freeze-thaw stresses, which could lead to compromised fire resistance of the assembly, which could lead to harm to persons.