It can be useful or it could be a waste of time and trouble too. I've tried it and sometimes lost the fish when they experienced a move from one tank to another, to another... and the resulting stress resulted in their demise.
IMO, the use of a quarantine/hospital tank is no different than placing the fish directly into a healthy reef aquarium and feeding garlic oil in their meaty foods for ~5 days. Learning what it takes and working to provide a healthy environment in the main aquarium, IMO, is the most important thing to focus on.
Importantly, if the LFS has the ability to place newly acquired fish in medicated water for a few days to a week, to kill parasites and gram negative bacteria, with copper and nitroferizone then the fish is already ahead of the game.
That said, recently I have found good success with newly purchased herbivorous fish by placing them in my "algae tank". This is the tank that also currently houses about 20 remaining baby Tomato Clownfishes that I sell for David Tea. It has a 2" bed of Oolitic LS, LR rubble and a few larger pieces of LR. It has a cleanup crew. It has a thick gowth of several types of macroalgae which the algae eating fish chomp on till they regain some meat on their bones. It has critters, amphipods and copepods, running around on the sand, in the algae and in the rubble.
Personally, I have found garlic oil, properly dosed in thawed meaty foods, to be almost a miracle for preventing the fish illnesses that once plagued every hobbyist.
Edited by Mark Peterson