To limit the probability that the performance of coatings applied to gypsum board to function as a vapour barrier will fall significantly below expectations with respect to water vapour permeance, which could lead to uncontrolled water vapour diffusion into insulated assemblies.
This is to limit the probability of condensation, which could lead to deterioration, which could lead to compromised structural integrity of assemblies acting as environmental separators, which could lead to harm to persons.