Whatever the motive, acquiring goods to bring back trip memories can be part of the purpose and pleasure.
I have included this from Laura L. apparently a Flea Market expert--
The name of the large flea market in Brussels is Le Grand SABLON, not "Le Grand Salon." It's important to understanding Brussels' history - the Grand Sablon is a part of town that leads down by slow degrees to what was, in Roman times, a place that hosted athletic contests, chariot races, etc. "Sablon" is from the French for "sable," which means sand, so it is appropriate that a part of town that leads down to a former contest area covered by sand would be called Le Grand SABLON."
Also, please don't forget the flea market in Madrid, which ALSO markets itself as "the world's largest flea market," which, if you take away the many adjacent buildings to Les Grands Puces and just include the stuff within the old Grands Puces, it actually is, as none of the surrounding areas of the flea market in Madrid are also antique markets.
Finally, a far better "real" flea market in Paris is Puces des Vanves, which is where (the dealers told me) people from Grands Puces come to buy their merchandise. I have been to both many times and if it's raining or cold, Sainte Ouen (Les Grands Puces) is much more comfy, but if it's sunny, you definitely want to be at Vanves!