They can then buy a few of each species from the same tank knowing that they will get along together in the new tank. It also helps to get a feel for how a fish will act with other species. Helpful is nice, but I'd take knowledgable. If the information is wrong, it doesn't matter how helpful the staff was. They need to determine if you have room for the new fish and if it is compatible with your other fish.
Some people mistakenly believe that if a store is clean, then the fish and animals are healthy.
Sweeping the floor does not help fish. If a fish is on the floor, the least of it's concerns is whether there is dirt or not. But don't assume that the fish are healthy. If the tanks are dirty, you may have something to worry about. On the positive side, the best stores are clean AND the fish are healthy, and the gerbils are running on their wheels and the birds are singing.