It seems that in almost every airport I'm in, there are luggage stores behind security. How often do people get to the airport carrying handfuls of clothes and needing something to put them in? I realize that there are limited cases in which they could be useful, but why are they so prolific and how do they remain profitable?
Even though the people who walk past are unlikely to be want to buy a suitcase right now, they are still the target demographic. How many other locations are there in a city where you can open a store where 100% of the people that walk past are people that travel by air, and thus the type of people that will be in the market for your products?
How many non-airport locations around an area like the San Francisco Bay Area would you need to open a store in order to pretty much guarantee that every person who flies internationally at least once a year walks in front of your store? A dozen? A few dozen? How about if you do it in an airport? Perhaps 2? (eg, SFO has 2 international terminals).
Some of the luggage shops you find in airports are for a particular brand. eg, there is a Tumi store in Newark airport. Even if that store was to never actually sell a single piece of luggage, they are building their brand and allowing people to see and try their product range before going home and ordering the same pieces from their website or from another store - all of which works in their favor.
The remainder of the stores generally sell high-end luggage which they would likely be able to ship to a home location if someone purchased something and was unable to take it with them or collect on their return trip.
At the end of the day, don't think of these as luggage stores, just think of them as high-end retail, marketing to a demographic that frequents airports (high wealth/travelers).
Setting aside the people who suddenly need more or different suitcases, an airport is one of the few places where you feel dissatisfied with the suitcases you have. While they're sitting in your closet, they're fine. You've used them for years and they work. But for the hour or two after you've packed them, lugged them from the car into the airport, and checked them, you know everything you hate about them: they're too big, they don't hold enough, they're too heavy, you don't like the wheels, the handle is too short, the color is old fashioned or the same as everyone else's, you name it. And you're surrounded by other people's suitcases to feel jealous of.
And now you have an hour to kill and nothing to do except pay too much for coffee or use up some of your reading material, and here's a store full of glorious suitcases. Maybe they have one that you were ogling somebody else use just moments ago. You can lift them, open and close them, work the handles, and compare in your head to the hated suitcase you just checked in. Maybe you'll buy on the spot and have them shipped home to await your return. More likely you'll take notes and order them online when you get back. If the store is part of a chain, they don't care which you do, this location can be more of a showroom than an actual selling things store. Of course, if you're carrying multiple carryon bags that you don't like, you might actually buy a replacement on the spot and swap things out and throw the old ones away. But the chain can make a ton of money if you never buy a full size suitcase there, as long as you buy one eventually. Heck, even if you don't go it, if you walked by and saw the name and saw that the suitcases didn't appear to be crap, they may have a financial case for that being worth it.
Not sure this fully explains it but the shopping area has certainly become an important revenue stream for many airports. I know several airports that have been redesigned to force passengers to pass in front of as many shops as possible, with S-shaped corridors and no short-cuts. Consequently, they sell a lot of things that most people don't need right there (and people buy them!). They are designed as a shopping location, not as a way to provide necessities so whether passengers already have a suitcase is not particularly relevant.
My local airport even advertises a service by which they keep what you bought until you return. As a matter of fact, I happen to know that they do sell a lot of bags so that in itself explains why these shops are profitable. Presumably, it works because airports are places where many affluent travelers have to wait or at least walk through every day.
It's straightforward ... people have an old, rubbishy, carry-on or roll-on case...
They stop at the samsonite shop, they buy a fancy new cabron-fiber one...
they transfer their stuff to the new one and throw out the old one, or, just take both.
If you watch the shops in question, you'll see this happening all the time.
Also very commonly (I've done this, actually a number of times) someone will have no roll-on with them; you might be carrying say a couple of paperbacks, a shopping bag, and a laptop or something - so you go right ahead and buy a new cabron-fiber roll-on.
Note that the shops you describe only sell carry on styles. If they sold hold-luggage, then just as you say it would be illogical!