I think you mean why do we need to define the cell zone? I think that is explained best in the docs:
This approach uses a ‘rotating frame of reference’ that modifies the governing equations in the rotating zone. Additional source terms that incorporate forces in the rotating reference frame are taken into account that simulates a rotation effect in the flow.
So we need to define a region of cells to apply the additional equations to. Whereas defining a rotating wall requires only a face patch to apply additional equations to (the differences are well documented in this tutorial).
Kind regards,
Darren