You may want to move heat from a central heating source to an adjacent space. Similarly, you may want to move cooled air from a window air conditioner to an adjacent space.

In other applications:

Move heated or cooled air from one room to another if there is a great difference in temperatures between the two rooms. i.e. one room is nice and warm in the winter. The room next to it, located over an unheated space is always cold. Simply blow air from the warm to the cold room.

Vent a room that always seems stuffy.

Move air in or out of an All Season room.

Vent a TV or family room with a smoker.

Balance temperature between rooms in a hotel suite or small apartment that is heated and cooled by a PTAC.