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 that support floors, this is to limit the probability of:
This is to limit the probability of excessive movement, vibration or deflection of floors, which could lead to negative effects on the psychological well-being of persons.