There are many applications where additional airflow would make an area more comfortable. In most cases, insufficient airflow is not caused by a lack of air handler cfm. The prime culprit is restrictive pressure drop caused by the duct size, length, and layout.
In-duct booster fans don’t generate additional cfm. They work by boosting the pressure so that more airflow reaches the area that needs it. Typically these areas are the furthest distance from the air handler, or where the duct size is too small.