The old Holley carbs did a good job when sized correctly for the application with only jet changes. Some of the newer carbs go a little overboard, thinking more has to be better.
Simply, the bleeds correct the fuel curve of the booster. And without any, fuel would tend to siphon fuel even after airflow stopped. Selecting the correct air bleeds and emulsion bleeds allow the correct amount of fuel to be mixed at any airflow through the carb.