[CMCDragonkai]

I believe that borders in the past existed mostly due to the ruling class that asserts control over resources. The peasant class are actually serfs to the land they are cattle to be corralled as you say. But ever since the development of nationalism and the nation state, borders are supposedly now useful to the the nation - tied to an ethnic group. But this might all just be in the mind regardless of biological affinity. Some people might feel more interconnected, others feel more egoistic.