fish suffer from a large menu of diseases (bacterial, fungal, parasites, worms, etc.), so naturally, QT
'ing is always recommended.
Outside of that, sometimes knowing the source of your fish can eliminate the need for QT
'ing. I have almost completely sworn off buying fish from LFS
and have recieved most of my stock from other aquarists/breeders.
There are some diseases that cannot be treated in your main tank as the meds will cause unaffected fish more harm than good. For example, FW
can contract TB, which is highly contagious.