To limit the probability of staple withdrawal, which could lead to the deformation or movement of underlay, which could lead to the displacement or cracking of finished flooring.
Where finished flooring is required to provide water resistance, this is to limit the probability of the ingress of water into the subfloor, which could lead to compromised structural integrity of the supporting structure, which could lead to an inability to resist expected gravity loads, which could lead to structural collapse, which could lead to harm to persons.