If you are striving for say an "Ich" free tank (and all my tanks are) then YES every item must endure quarantine. (there are a few exceptions of course, Mandarin fish comes to mind)
QT gives you time to assess the health of a new item, lets a new fish adapt to the trauma of transport and eat without competition while it gains stamina back from what must be a terrible experience for a fish.
On that note, I did stock my first Nano without QT and got really lucky initially and then introduced a Royal Gramma that brought in the "Ich" 8 weeks later I had the problem fixed but it was last time I put a new item into an established tank.