Not just a place to hang out. It isn't a good habbit to just place new fish into your display tank (DT) without first monitoring them and making sure they aren't sick. If you don't have any fish in your main tank then a QT tank isn't necessary.
If you do have fish already and you are wanting to purchase new ones then you should have a QT tank. Most of the time if a fish is sick you won't notice it at the store, or for a few days for that matter after the purchase. The symptoms won't come around until further down the road and by that time all of your other fish will have gotten infected.
If you do purchase fish that become sick, with ick for example, you can use the QT tank to treat the sick fish. Because most treatments for ick will actually kill inverts. If you can afford it QT tanks are great ideas and should always be used - but if you are poor like myself and only have a few fish they aren't necessary (unless they get sick).
Hope that helps you out.