To explain a little more, you cycle a tank when it is new to get those good bacteria in it. If you don't have them, the waste from the fish will build up in the tank and kill them.
As mentioned you quarantine things for sickness and prevention of sickness. When fish are sick, you separate them so that you can treat them and protect your other fish. When you buy new fish, you can put them in a special holding tank apart from your current fish to observe them and see if they are sick. You can also treat them right away or when you see signs of sickness.
The pinned threads are very helpful to read.