As opposed to running nothing at all, e.g. air box, filter. There is a significant gain to be had from running velocity stacks, especially on an outboard where almost all the intake air comes from behind the carbs. Having a large radius on the outside of the stacks is critical
Even from a non scientific point of view it is clear to imagine how the air flow compares between a sharp 90 degree edge in the case of no stacks to a smooth radiused curve with stacks.