Balance tubes were popular with the old Zenith carbs (Similar to Kadrons) on early Porsches and VW engine conversions. Mostly, the balance tubes were used to provide an adequate vacuum signal for the vacuum distributors that everyone used at that time (the 1950’s and 1960’s) because centrifugal-advance distributors had not yet become popular. If you were to use the dual-throat carbs and intake manifolds that we have today with a vacuum advance distributor and simply connected the disti vacuum signal to any one of those 4 intake manifold runners, the vacuum signal would pulse (spike) once every other crank revolution. The disti would go nuts, pulsing all over the place and never give a good spark signal. The engine would never see adequate spark advance to run properly.
The cure is to connect all of the intake manifold runners together to smooth out the vacuum signal (balance) to provide a smoother spark advance signal. Some older disti set-ups (and a lot of racers, because they run at wide open throttle more often) plumb a vacuum buffer tank into that system. That is just a small sealed can connected to the vacuum line to provide a vacuum reservoir to the system to smooth out the vacuum signal at the distributor as well as slow its’ action.
All that said, some single-throat dual carb setups like to be connected together at the intake manifolds because the carbs are single-throat and benefit from the vacuum effects of merging the intake ports and the better merging of the fuel/air mix. Think of this as not pulsing the carbs by starting/stopping the air/fuel mix for one cylinder, but providing a more balanced intake flow being drawn by all four cylinders and both carbs. It makes the whole system a bit more efficient, but that only works on single-throat carbs. If you are running carbs like dual-throat Webers or Dellortos and/or are running a centrifugal advance distributor, you will probably not see a benefit to installing manifold balance tubes.
I hope this is not confusing. If so, ask questions and I’ll try to clarify.