Each NFT for Farms, Mines, Troops, etc. has random quality characteristics that are obtained when crafting it.
For buildings, this affects the percentage of resource extraction from the maximum possible - the higher the quality of your NFT, the more resource it produces at its level. As you level up, the quality doesn't change, so more "good" NFTs are always much more valuable, as they provide more and more resources with their upgrade.
So a Farm with a quality of 90% will give 90% of the standard amount of resources on each level. So higher-quality NFTs are much more valuable, as they bring more benefits for the same cost of improvement and maintenance.
For troops, this parameter is called "stamina" and determines the ability to perform better in battle. Special amulets and power orbs increase the chance to craft better buildings and troops.