To limit the probability that the insulation will settle, which could lead to incomplete coverage by the insulation, which could lead to excessive heat loss through portions of the building envelope.
This is to limit the probability of excessively low temperatures of interior surfaces or within assemblies, which could lead to condensation.
This is to limit the probability deterioration, which could lead to compromised structural integrity of assemblies acting as environmental separators, which could lead to harm to persons.