The word "quarantine" is derived from the Italian word for 40, "quaranta". It was the amount of days a ship had to stay anchored outside of Venice, Italy during the waves of the plague that killed hundreds of thousands of people in the late middle ages. Quarantine was established in order to assure that there were no rats on board carrying the disease and that nobody already had the disease.
It's a good metaphor for what we use quarantine for with our aquariums; we watch and see if anything bad happens. While we often use medications to control unseen disease in our new fish (i.e. a formalin or copper treatment) the idea is to keep them separate and watch carefully. Other benefits of quarantine is allowing the fish to learn to eat and become accustomed to an aquarium environment and also to decrease stress levels so that the immunity of the new fish can be high when introduced to the display aquarium where treatment is almost always more difficult. Stressed out fish are more likely to get sick from opportunistic infections which are present in most tanks, think humans getting a cold in winter time or when under a lot of stress at work.
Some fish can live a long time. Tangs in the wild generally get to be more than 30 years and 20 year old percula clownfish in home aquariums are not uncommon. Unfortunately, at some point in most aquariums there is a disaster, a tank crash or power outage, but most often a disease which wipes everything out. Standing by and watching helplessly as a diseases progresses through a display tank is a tough way to learn a lesson about quarantine.
Honestly, we at healthy aquatics spend most of our time helping folks who on impulse bought a fish that went straight into their display tank and caused a problem.
The goal of our quarantine description is to give a straightforward guide to ensuring your fish go into the display tank happy and healthy.