Not to jump on the bandwagon but there are honestly 100 reason to do it none not too. But I read one article about this subject that made the most sense to me and I cant find it. Aside from the entire "dont bring unhealthy pest/animals into your display" argument. If you simply follow the experience your new fish has taken over the last month or so that led to it being in your hands in a bag at your house, think of the QT as a "spa for fish" believe me your new fish is stressed out, and not anywhere near its healthiest state. The QT allows you to let this new guy, relax, get acclimated to your food source that will be offered daily, and simply get healthy before making that last change to his new forever home. By allowing your fish to "re-engergize" there slime coat will thicken, they will fatten up and overall just be far healthier than they are when you bought it.
This new state allows your new fish to then handle the stress involved in making that last step, and believe me its stressful on every fish to be dumped into a new tank with new mates.
Plus you can treat for illness if any is spotted. Many "issues" experienced by new fish is due to being introduced into a new tank while they are basically as a state of low health. Give your new fish a chance, since using a QT new fish additions have gone off without a single issue.
Oh!! one last trick Ive used, when you finally move your new Buddie to the display, do it when the light are off for the night. I have no real idea why, but since changing "when" I add my new fish, when the lights come on everyone acts like they been together for years. it works.