To limit the probability of an inadequate grade or thickness of sheathing relative to framing member spacing and edge support, which could lead to an inability to resist expected gravity and lateral loads, which could lead to excessive deflection, which could lead to damage to roofing or walking surfaces.
For floors and elements supporting floors, this is to limit the probability of excessive movement, vibration or deflection, which could lead to negative effects on the psychological well-being of persons.