Why!
That is the most common answer I hear when talking to people new to the fish keeping hobby about quarantining any fish before placing them in your established aquarium. Most people argue the fact that they cannot afford the cost of such a tank, but if you applied " Murphy's law " in that if something will go wrong! it will and at the worst possible time.
So just imagine that you setup a beautiful tank with say 10 Rainbowfishes at a cost of $12 each, that is an investment of $120. One day you go down to the pet shop and buy a couple of fish that you've been wanting to get for a long time but have been unable to find. So you buy them and add them to your display tank that day since they look so good. Everything seems OK!, But you wake up the following morning to find your new fish looking and behaving differently, not having much time you make a mental note to take a better look at them that afternoon and then rush of to work.
Arriving late from work you notice that one of the new fish is dead and the other fish gasping for breath and swimming slowly in the corner of the tank, but! to make matters worse a few of your other prized fish also look unwell. By this time the pet shop has closed and you have to wait to the following afternoon to get help, You come home that afternoon and find that you now have lost the other new fish as well as another two fish, you add the medication that the pet shop sold you and you end up saving the rest of your fish.
That was a loss of $48 which is about the cost of a small tank that you could use for quarantine purposes. That is why it is a good practice to have a small tank for quarantine purposes. The quarantine tank allows you to keep a eye on the fish that you buy for a short period of time usually one to two weeks would be sufficient. During the last few days of quarantine you should add water from the tank that the fish are going into, normally a quarter of the holding tank size eg; if the fish is being kept in a 60cm x 30cm x 30cm tank you would add 12lt of tank water to it.
This would give your fish time to adjust to the water conditions of your main tank and minimizes the stress/shock that the fish could suffer when you add it to your main tank. There are other reasons has well most of them relate to diseases that the fish could be carrying. These are explained in more detail in the Disease section of this website.