To exempt openings for electrical and similar service outlet boxes from the requirements of Sentence 9.10.5.1.(1), which would otherwise not permit openings in rated wall or ceiling membranes, if certain conditions are met [such outlet boxes are tightly fitted].
This is to limit the probability that the openings will contribute to the spread of fire within the wall or ceiling space, which could lead to the premature failure or collapse of the wall or ceiling assembly, which could lead to damage to the building.