My reason for using a spacer, specifically an open spacer, is to allow all cylinders access to all for barrels of the carb. Since the stock intake manifold is a dual plane manifold, each set of cylinders only had access to basically a 2 barrel carb. By adding an open spacer I have allowed access to all four barrels to the entire motor. Thereby improving efficiency. The spacer did fit perfectly, only difficulty was bending both the steel vacuum line for the brake booster and the steel fuel line. I noticed an obvious increase in throttle response, and that the elky gets up to speed quite a bit faster. What I have yet to test is how it has effected my fuel economy.
The two-plane manifold is designed for the best distribution for all 4-barrels of the carb. The spacer will help the mixture "turn the corner" from the plenum, into the runners etc, by increasing the area of the plenum. If yours performs better now, the spacer must have helped a bit, sort of like a high-rise manifold would.