Furnace Draft Inducer Repair: is previously installed back plate necessary?
While those answers may apply to some degree, I know the plate does create a restriction, which causes a slight vacuum in the blower section.  This vacuum is used on some furnaces to actuate the diaphragm pressure/vacuum safety switch.  If you have a diaphragm switch hose leading from the blower housing, then you'll find the furnace will not start gas flow unless that plate is in-place and creating the vacuum signal for the diaphragm safety switch.  Ask me how I know...  duh.