TL;DR: When you need to decide if a certain resource field or culture point building should be built before the other (or built at all), choose the one that has a lower break-even point. This means that your investment will return profits faster.
Early in the game, you are likely going to be valuing Culture Points more, than late in the game. Which means that early on you might want to be building buildings that might not have any useful utility value on their own, but only increase your base CP creation. Some buildings and levels are better for that, than others. Similarly comparing resource field upgrades to buildings is rather common question early in the game. Late in the game when you might value CP much less than previously, when you already have all the villages you need and want, most of the building levels become unattractive investments outside of their utility value. Some buildings don't have a break-even point, when the increased CP gains are smaller than the increased population and crop consumption.
Below break-even points for resource fields and buildings can be used for this decision making. Tables list how many days it takes to pay back their initial costs for increasing each field or building level, with different 'Resources to CP' exchange rates. In other words, how long you need to wait until you theoretically have more resources and CP compared to a situation where the investment would have never been done.
travian_kingdoms_buildings - Google Drive -> 'Building Prod + CP' tab.