Water changes can be fairly traumatic to aquarium livestock. The water level in the tank drops significantly, exposing corals and invertebrates to air for at least some period of time. Once the water has been refilled, the livestock are then exposed to water of a different salinity, pH, temperature, etc. On top of all that, the water change often stirs up detritus and other gunk that had previously settled somewhere in the rocks or parts of the equipment. By disturbing them with a water change, they are now free to cause problems in other places, such as clogging up pumps or landing on aquarium livestock. Of course, you could always catch that debris with a filter sock or some other form of mechanical filtration.