To limit the probability of inadequate rigidity of underlay panels, which could lead to the deformation or displacement of underlay at joints, which could lead to the cracking of ceramic tile.
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.