The spacer helps to insulate the carb from the manifold which in turn minimizes the increase in fuel temperature within the carb.  Hot soaking of the fuel within the carburetor after shutting off the engine is also reduced.  A secondary reason is to increase the distance from the carb to the plenum base within the manifold which helps the fuel distribution and in turn improves the power production. As a general rule, increases in spacer height help with torque production.  Here’s a link to a test where different designs of carb spacers were used on a four barrel intake that had dramatic effects on the power production versus no spacer at all.

http://www.eatonbalancing.com/blog/2012/09/29/carburetor-spacer-testing/