Van Rijn asks Dalmady:
"'You wonder why I tell you what you know, ha?'" (The Van Rijn Method, p. 523)
The real reason is that the reader needs to know it and, if the omniscient narrator directly imparts information too often, then the narrative becomes an essay. Of course, van Rijn has his reason within the narrative: he is checking the facts with the guy who was there. How often does Flandry "run barefoot through the obvious" for the reader's benefit?
We recently read a list of Solar Spice & Liquors cargoes. Here is another: bluejack. However, bluejack is shipped not from Earth in Company ships but from Suleiman by freighter. It is a Suleimanite plant used by the natives as a spice and tonic. Suleiman is subjovian and its life is based on hydrogen, ammonia and methane. Thus, bluejack can be sold to other hydrogen-breathers if they have the special biochemistry that benefits from its complicated blend of chemicals corresponding to hormones with synergistic effects. Because markets are few - mainly on Babur - synthesis would be more expensive than paying the Suleimanites in trade goods to bring bluejack to Company warehouses.
Thus, while we understand the appeal of van Rijn's cargoes from Earth, we have to accept that bluejack has the same appeal for Suleimanites, Baburites and a few other hydrogen-breathers. Such is the nature of an interstellar economy.