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. The “Commercial Resources” section tells you what resources you have access to in your empire that you can export for commercial profits. You can also see what resources you import, which is very useful for fractions like dwarves, who need specific resources for manufacturing. 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. 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). 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. The commercial status always shows 1 out of 3 options. Existing trade agreement, no trade agreement in force, but there is a possible way, or no trade agreement and no road possible. When you hover over the symbol, it always shows you the potential income of a trade deal, unless you play as a political group that cannot act such hordes or Orcs.