The simplest way to think about it is this...
By using three 2 bbl carbs that open at different times and are jetted differently, the total airflow and A/F ratio is optimized so that it is never oversized at low rpm and never flow restricted at high rpm.
It would be like if you could use your 600 cfm 4 bbl at low rpm and then switch to a 750 cfm at high rpm.