There are usually clauses in the code that tell you the minimum strength of a floor plate to transfer the axial load from higher strength columns.
You can get away with the lower strength at the floor plates due to the additional confinement provided by the adjacent floor.
Other than this floor concrete strength is usually only dictated by durability requirements. There are other reasons such as shear strength, abrasion resistance e.t.c. but these are less common.
