more and more efficient airflow, especially at lower rpms.
the cylinders do not fire at the same time, one fires then the other, on a 180 degree cycle.
with a dual carb setup, the air rushes in, then stops, then rushes in, then stops, very very quickly, from one cylinder/carb to the other.
with a single carb, there's a constant flow of air coming through the carb (hence the name, constant velocity kit) and this provides a more efficient flow to the cylinders especially at lower rpms, therefore boosting power. also, the larger bore increases overall flow.
that's the jist of it anyways. there's loads of posts on the single carb setup, search some here.