Imagine setting up a very nice tank, keeping healthy and happy a fine assortment of fish costing a total of well over a $100 or many times that, enjoying the peculiar habits and personalities of some of your more expressive fish...
Then you buy a $4 fish, plop him in, and a week later 4 in 5 of all your fish are dead.
I've heard of exactly that happening several times.
A QT tank is important.
Not only does it give you time to make sure your new additions are disease free, but it also gives the fish time to adjust to how you take care of it(food, maintenance, etc.) before you introduce it to the stress of cohabitation with the established occupants of your main tank.