What do cities and/or cores do?

[Cigaretteofdeath]

Might be a stupid question but beside the capital i do not see the purpose of a city or know what a core is or does. Would be nice if someone could explain. Or is it just so that a city can quickly change a city to its capital to "dodge" being conquered if the current capital is closer to the enemy? Also i do not understand when and how cores appear? Do they always turn into cities directly?

[Relonee]

When a city is a core then there can be no revolts and the nation who have a core on it can revolt from it.