fuel has a very good point
apart from the various coral pests, the water that your corals carry with them (internally or on the surface) into your tank, even though the bag water is discarded, still can carry diseases and parasites for fishes. i've read of many reefers who put their corals through 8 weeks of qt to get rid of the risk of ich for their fishes. this applies slightly more to lps who would carry the previous tank's water in them into the new tank compared to sps which deals with mainly water on the surface but i do not mean lps should be qt and not sps