Another advantage of a QT is that it helps in the acclimation process, especially if your main tank's water chemistry greatly differs from the water in which the fish were initially kept. When you get the fish, have the QT match the water chemistry of the source water as closely as possible. While the fish are cooling their heels, or I guess in this case, fins, in isolation, you can gradually adjust the water chemistry to that of the destination tank.
Also, many fish may have been weakened during the transportation process and may not compete successfully for food in the main tank, or otherwise may be picked on by stronger tankmates. The QT isolation gives you a chance to fatten them up.