Boundaries are borders. Borders mean something if, and only if, you are willing and able to enforce them.
The same people who demand no borders for the movement of people or capital will fly off the handle if you challenge their social dogma (violate social boundaries), question why they go after your economic or private life (violate your economic or social boundaries), and gainsay their philosophical assumptions (violate their mental boundaries).
This does not need a long explanation. Borders bring Civilization, first by their defense and then by their respect. Barbarism, especially Empire, comes from predatory behavior of the sort that's done because the predator cannot or will not sustain themselves by themselves for themselves. (For all you r/K types, this is r-type predation because they go where the resources are and go all Locust Crusade to get them.) There is no reasoning with them; you not only build and man that wall, you get ruthless with them (regardless of the skin they wear) when found. There is no reasoning with the ravenous hordes of savages and their sweet-talking sorcerers who beguile their way to power. There is only war. War to the knife. War to the hilt.
Civilization exists only when there are hard borders held by hard men appreciated by loyal wives and dutiful children. Softness is taken as weakness, time and again, and the hordes win when the hardness is gone. Every single fucking time.