The way that things SHOULD work is that your fish store gets stock in and isolates it for two weeks. Few do. Especially with really good stock, they look healthy and attractive and will get a good price if showed off and sold quickly.
Good fish retailers know that the change of tank and water conditions will affect all fish and condition, colour and behaviour will take a dip, so they don't put new stock on display straight away. They give the fish a chance to adjust, get it's immune system up to scratch (stress on fish will knock their ability to resist parasites and disease) before putting them in the show tanks for you to buy. You then buy a fit and heathy fish but put it under yet more stress by changing the water, tank and competition (other fish in the tank/pond). Any infection, parasite or genetic weakness that is not a problem normally will come to the fore, escalate and possibly transfere to other fish.

Isolating newly bought fish gives you the advantage of assessing their overall health, any "problem" fish, such as fin biters and allows you to see rampant defects that kill the new fish without affecting your old fish.

Once the fish have gone through a week of isolation and you are confident that there are no problems then start to transfer 3 or 4 cups of water from the isolation tank to the main tank and 3 or 4 cupa of main tank water to the isolation tank three of four times a day for another week. Any signs of illness or distress should be taken seriously.

If you want to keep old stock healthy then isolation of new stock is essential, no matter what the person selling the fish to you said.
And, yes, every time.