City is the main way to increase your Realm's population.
In the future, population will be used to determine capacity and productivity. So even if you are not competing for MAGIC Rewards, it will be important.
Each time you build a City, you are given the ability to boost the Realm you built the City in. The boost will only apply to the current month's Metrics.
If you build 5 Cities, you can use up to 5 Nourishment Collectibles to boost your population for that Realm. The next month, the population of your Realm WILL NOT include the boost.