Political groups can agree on trade through diplomacy. A trade agreement between two factions is because of the Lore. If you were an elf, you wouldn`t negotiate with a vampire. Animal men, greenskin and chaos warriors cannot act. Maintaining friendly relationships with other groups offers a number of advantages. First, the trade agreement and the non-aggression pact that guarantees the security of your country. The Military Access Treaty allows your armies to move freely on the territories of another faction (which, without it, is treated as a violation of borders and deteriorates your relations). Tilea and Estalia are also very difficult to obtain as business partners for reasons I can`t understand. Note: The Tomb Kings are the only faction that actually uses resources for their mortuary cult mechanics (Edit: a similar mechanic called forgery has been added to the dwarves in Warhammer 2 only since this response was originally published).
For all other groups, resources only increase the value of trade agreements. So you`re generally right to think that these resources are not actually used by the trading partner, but not in this particular example. I have never seen evidence that more commercial resources would increase the likelihood of someone agreeing to a trade agreement. The diplomatic system is very veiled here. I have seen that groups with 200-plus relationships are opposed to profitable trade for them and that groups with 5-strong relationships accept the same trade. Each resource you export brings a sum of money per train. The revenue you receive from the resource can be verified by indicating the resource listed in the Diplomacy with the Mouse tab. To check how much money you get from trading with a group, you have to float on the icon of the trade agreement, next to the group given in the diplomacy tab. On the left (on your side), the interface shows you how much you earn with the trade and resources you export to your trading partner. The right side shows how much money they earn and what resources you import from them. It also depends on the characteristics of the political groups. A group like Kislev, which has passive quality, is by no means to act.
Trade can take place between two political groups in campaign mode. Trade agreements can be concluded on the diplomatic screen, allowing political groups to exchange trade resources and generate additional revenue for both factions. Although diplomacy is often limited to other factions of the same race, you should take a moment to learn more. Diplomacy allows you to conclude trade agreements, non-aggression pacts and even military and defence alliances (if your relations are good enough). In the case of fractions of the same race, you can also try to integrate them under your reign. In addition, by clicking on a group in the tab on the right, you will see a list of contracts and agreements binding the group, as well as some information about the commercial goods they have (the ones they import and export).