Are you talking about the .090-.100" thick plate, shaped like a base gasket?
If so, that is to ensure the edges of the gasket seal when the carb is mounted to an intake with a dual squarebore/spreadbore carb pattern....otherwise, there is a good chance of a vacuum leak occuring at the carb base on eather side of the area where the wide secoindary barrels section is on the intake manifold.
It doesn't need to be used if the intake manifold is a square bore pattern only.