To limit the probability of excessive water loading on below-ground assemblies, which could lead to water ingress from the ground, which could lead to deterioration, which could lead to compromised structural integrity of the separators or of elements supported or protected by the separators, which could lead to structural failure, which could lead to harm to persons.
Where floors are supported on ground, to limit the probability of reducing the soil-bearing capacity due to saturation, which could lead to structural failure, which could lead to harm to persons.