@gwand has described what's in those stores, and I can only drool.
Online can fill the tanks of those of us who like less common fish groups. If you drift into specializing in a region or a Genus, it's the only way. If I had access to the Wet Spot, or Toyin Ojo's place in Florida, I could spend a lot of money. They have fish that are what I want to keep, and don't see here.
For less rare things, I still think there's value in seeing the fish and evaluating their health. If you buy neons online, they could come from a tank with 50 corpses in it, and be the survivors of a shaky shipment. You have no way of knowing. Study the tank in a store, and even if the corpses were removed, a self trained eye (one that has looked at fish disease books in detail) can see problems and move on. There is an enormous amount of trust necessary for online buys, and if you trust the seller, you're good. If you don't know the seller....