Mining is one of the most important things. While prospecting, you will gather with your NFT an amount of various resources. Since building your army requires a lot resources gathered from mining, this remains one of the most important category.
Every NFT in this category, marked with an iron symbol, will bring different resources. Some NFTs will bring in addition coal, gold, diamonds, or other rare resources.
Having your own land will bring you passive resources if the NFT provides a bonus for this category of resources.
Mined resources will be used as materials to craft various things from your discovered blueprints, and other purposes.