To limit the probability of concentrating water run-off at the perimeter of the building, which could lead to excessive moisture loading, which could lead to water ingress, which could lead to deterioration, which could lead to compromised structural integrity of the separators or of elements supported or protected by the separators.
To limit the probability of soil erosion, which could lead to compromised integrity of soils supporting the building, which could lead to structural failure, which could lead to harm to persons.