... QT tanks are always essential and its a good habit to get into. The (3) biggest reasons to have them are:
1) QT new acquisitions for a few weeks before transferring into main tank. THis is to ensure that your new pet isn't going to infect your DT with a parasite or disease. At that point your DT would be down for another (6) weeks QTing all livestock while the unwanted disease disapates.
2) QT to medicate a sick fish
3) separate an overly agressive addition to the tank.
From what I've heard, a small filter, and a heater are the basics for a QT tank. You do not want LR
as it may be contaminated (ie copper) no substrate for the same reason. A light is generally optional, as less light usually means less stress.
A QT tank thats set-up is nice to have, in case of an "emergency" evacuation or medical issue that needs immediate attention.