The Empire`s income comes from a variety of sources. The most important taxes are applied to the entire population and just behind them you have the trade. The income from slavery depends heavily on the amount of slaves you have enslaved during your conquests (it is a simple multiplier of basic income in the provinces). In addition, you can get an annual tribute to the conquered nations and rewards for the conclusion of the mission that was ordered by the Senate. The next tab shows the details of the trade. If you have a trade agreement with another country, you can see here a list of resources imported and exported with their prices. Imports of construction resources are very important in the later stages of the game, when the larger structures available do not need easily accessible resources such as marble for example. 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. 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.
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. 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. Animal men, greenskin and chaos warriors cannot act.