What is the difference when you analyse a structure for seismic load with floor diaphragm and without floor diaphgram.
Having the rigid floor diaphragm specified allows you to model the lateral stiffness due to the diaphragm which you will not get if choose not to include it. Moreover the rigid floor diaphragm has the ability to automatically figure out the torsion due to eccentricity between center of mass and center of rigidity and that effect is automatically included in the analysis when rigid diaphragm is included.