It's the ghost of merchantalism, really. What matters to a nation or empire historically is putting more stuff - goods, ships, people - "on the books", and call

It's the ghost of merchantalism, really. What matters to a nation or empire historically is putting more stuff - goods, ships, people - "on the books", and calling that the wealth, but annexation and political incorporation as the way of accomplishing this gradually fell out of favor as we entered the modern period and turned towards productivity metrics. But you can create a scenario where your accounts - you being "whomever holds political power" - are inflated relative to others by enforcing favorable borders and terms of trade, so that you wield more productive forces relative to everyone else. And that's been the strategy of industrial-colonial countries. If you are for global productivity, rather than local wealth, then open borders are the way, but they come with all kinds of other political challenges since they put the world on a course to unification more generally.