Government's main purpose is to establish a social order for a population too high to accomplish that through the natural human facilities.
That's really it.

Everything else is just flavor added by each specific culture. Different groups have different ideas on what it is that they need for social order.
The most modern governments thus try to rule loosely in order to allow for those differences to prosper... as evergrowing populations lead to evergrowing diversity and far less social cohesion.

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"It's never over... BOY!"
The Tall Man, Phantasm III