A refugium provides physical isolation, is still connected by plumbing to the main tank. The QT is completely independent water and equipment, so disease can't travel back into the main tank.
If you keep your qt running all the time, you have to keep providing an ammonia source. You can feed the empty tank, or keep a small population of critters (like snails) that you don't mind loosing if you end up having to treat a diseased fish in there. If you don't keep your QT running all the time, you'll have to scramble to keep the ammonia levels down when you need it.