Anthropology.
There would be intense scientific interest in watching a medieval society evolve. Human knowledge and understanding of such societies being limited to the lens of history and various archeological discoveries etc. But just as is the case with the few remaining archaic cultures on Earth history has also shown that contact and interaction between advanced and primitive cultures can have devastating consequences for the latter. So while there is great interest in studying the peoples of the planet there is also a lot of public pressure on the teams not to screw things up. Your advanced culture does NOT want to repeat the mistakes of the past.
On top of that there is a purely selfish motive i.e. the near certainly that introducing themselves as they really are to the local cultures would ruin their one and possibly only chance to observe a 'pristine' medieval culture developing.
So they devise a plan. They use these once a year low key 'trading missions' to interact and observe. And it doesn't hurt that the hand made goods the 'traders' receive in excahngeexchange for their own goods would be worth a lot of money to Universities for study purposes and to collectors.
For that matter the covert recordings they make during their trips would not only be of major scientific interest but once edited their sale to media outlets for public consumption would probably pay the cost of mounting the expedition in the first place! That being so your 'merchants' are all trained anthropologists, linguists, historians and 'minders' appointed by the Government to make sure none of the 'rules' are broken.
Finally as someone else suggested the yearly visits also allow the teams to recover data from hidden recording devices located at major sites of interest. All of which have been busily compiling data since the last teams visited.