They are playing the long game
They have a several century long plan to groom this civilization from medieval to interstellar age in order to become a powerful ally to them. The beneficial trading is part of that plan to inconspicuously boost the prosperity of a specific culture which will in turn affect the history of the planet in the way they intend.
How can they pull that off? They are just that good historians. They studied the history of thousands of pre-space worlds and created sophisticated computer simulations to model historic developments with astounding accuracy. So they can predict how the future of a civilization can be shaped through small interventions like that and can use it to shape their future in any way they want.
For example, the aliens predict that in a couple generations a certain trade-good will become very important. Whatever nation can produce it most efficiently will become the dominant superpower in the next couple centuries and their sociopolitical philosophy will shape that of the whole planet. But right now this good seems rather unimportant. So the aliens start to buy that good from their preferred nation now, so they get a head-start in building the know-how and infrastructure for producing that stuff.
Why not just hand those nations anything they are going to need directly? Because then they would become complacent and reliant on the aid of the aliens. In the long term that aid would make them weaker, not stronger.