Before I came to Japan, if trading company had meant anything to me it probably would’ve been a dodgy wheeler dealer like Trotter Independant Traders. If the number of times “trading companies has come up in my Business English teaching in Japan is anything to go by, though, in Japan they are a whole lot more important and a whole lot more respectable.
The historic reasons are to do with a lack of foreign currency after WWII, but since then it is more the expertise and confidence to deal with foreigners and traders that they have monopolized. The fact that other large Japanese companies won’t usually do business with people they have no prior relationship with also gives them a role. Or so I have read, having, as with most things on this blog, no personal evidence to go on. As with all things on this blog, however, alternative theories of any type always gratefully received.