Saltwater fish are not only quarantined to prevent passing on any diseases or parasites to the other fish in the aquarium but to give them the best chance of survival when adding them to your main reef tank.
These fish have more than likely been caught in the ocean, transported to an importer and then survived the journey to your country. They are then sent off to wholesaler who provides pet stores with their stock. We then buy them from a pet store and take them home. If nothing else, these fish need to recover for an extended period before having the energy and strength to cope with being put into a main display tank.
When fish are stressed, their immune systems are weakened. If they are then added to a tank that may have a disease or parasite, then their chances of them having the ability to fighting it off are not very good. They may also end up succumbing to tank bullies who can be quite ruthless in establishing their dominance to a newcomer.
Another reason to quarantine fish is to get them used to your tank’s parameters and the type of food you feed your fish. Sometimes you may have to try a few different types of food, frozen, dried etc. before coming across something they will eat but with a bit of persistence, they should start eating the food you feed in the main tank.