In the south it is not uncommon for ponds to have DO problems below 1-2 feet at certain times. This forces the fish to all be in that small portion of the pond (top 2 feet). Any water movement that helps avoid that situation is beneficial. Fountains are not the best by a long shot at fixing water quality problems but they are much better than nothing when the fish are all crowded in that limited area. As Bill notes it is much better/efficient to move water with air than by a motor.