To limit the probability of unlevel surfaces underlying sill plates, which could lead to discontinuous support for sill plates, which could lead to an inability to resist expected gravity or lateral loads, which could lead to excessive deflection or failure of sill plates.
For floors and elements supporting floors, this is to limit the probability of:
This is to limit the probability of excessive deflection or vibration of floors, which could lead to persons losing their balance, tripping or falling, which could lead to harm to persons.