Carb spacers only do one thing, increase the size of the plenum area. Larger plenums = higher RPM use and higher RPM power curves (less low RPM torque). Small plenums = lower RPM use (below 6,500 - 7,000 RPM) and greater lower RPM response (velocity and mixture), for quicker power and torque curves. In other words, the larger the plenum, the "lazier" the air (and fuel) is inside the plenum it so it requires higher RPMs to create the velocity needed to get that air/fuel mixture moving the directions it needs to go (down the runners).
Ever see any factory engines (including 99% o muscle cars) come with carb spacers or large plenum intake manifolds? Nope. Most use dual plane intake manifolds rather than single planes because street engines are in lower RPMs 90% o the time and use "torque" for acceleration rather than "horsepower" (which only comes into play above 5,250 RPM). Unless you are operating at or above 6,000 - 8,000 RPM and are using a single plane intake manifold, you "shouldn't" need a carb spacer. The intake manifold manufacturers aren't stupid. If they wanted a plenum to be larger, they would design it to BE larger. One of the only manifolds where a spacer is commonly used are small block Chevy Edelbrock Victor Jr's because so many guys use those on hot street engines (and mild race engines) that see a lot of upper RPM track time, yet still a lot of lower RPM street use. That manifold works well on the street, but at the track, in much higher RPM use can use a little bigger plenum (because it isn't a "serious" race manifold like a Super Victor is), so carb spacers are pretty common on those, AND to top it off, Edelbrock KNOWS this and offers that manifold with a 1" taller plenum right out of the box so you don't have to buy a separate spacer IF you are going to be running in the upper RPM range more often than normal. Again, manifold manufacturers know what works and what doesn't, and if they thought they needed spacers, they would modify their manifold design to have it, (like Edelbrock did), or would recommend one for certain applications (higher RPM use, larger cubic inch engines, etc), or they would supply you with one. But the Victor Jr is the only manifold I know of that has that option.
Spacers are typically used on circle track engines and drag engines that operate at peak RPMs most of the time IF the cubic inch displacement is greater than what that manifold is designed for, or the RPM range is higher than what it was designed for using a given runner size of the manifold and heads.
Dual plane intakes shouldn't use spacers because it defeats the purpose, and again, slows down the velocity which is needed to create good low-end torque, which is what moves the car, especially street cars that don't launch off the rev limiter like real race cars do at the track, or operate in their peak power RPM all of the time like true race cars do.
Most single plane manifolds don't have any provisions for a manifold vacuum source, so some guys will run a spacer that has a vacuum port on it when running a single plane manifold equipped street engine so they can run their power brakes or have a PCV line, etc. What we do rather than messing with the plenum size by having that spacer is to just drill and tap the manifold (before you install it), for a vacuum port. There are almost always places on manifolds to put one, either on the side of the plenum (where we prefer to put them), or in one of the runners.