Boundaries are pervasive throughout the biological realm. Start with the simplest living thing: the cell. The cell is defined, first and foremost, by the cell wall. Is the interface between the interior of the cell and the environment beyond the cell. In particular, it regulates what passes through the cell wall, and what must be kept out if the cell is to survive. And what do we do if there are some cells, say those of pathogens, that we want to kill? One way to kill them is to burst their cell walls by subjecting them to high heat. No cell wall, no living cell.
Let us move up to the level of multicellular organisms. We still have the rough equivalent of the cell wall, but now it is more complex: skin, bark, thorns, roots, hard shells, exoskeleton, the surface of leaves, mouth, nostrils, anus, the immune system. All function to bring into the organism things helpful to its flourishing, and keep out things harmful to it. And to destroy an organism, we breach its defenses: with poison, with a flood of harmful microorganisms, with a sword or a bullet.
Similar processes take place at the level of groups of organisms. An ant colony builds its nest underground, to make it harder to find and access. It creates a limited number of entrances and exits, and has soldier ants to guard those. Ants can detect by chemical means which other ants are from the same colony and which are not, and will try to drive away outsiders from their foraging grounds.
Human groups are not different in this respect. Consider the Miami Heat: OK, they are presently getting spanked by the Spurs. But think of how much worse they would be doing if the team was unable to regulate who could show up to play so that, say, Bob Murphy and I could check ourselves into the game at any point we wanted. Factories fire workers who try to sabotage the assembly line rather than producing the desired product, restaurants ask people to leave if they try to occupy a table with no intention of ordering any food, and the swingers club must eject those who arrive not to swing, but to read to the writhing crowd biblical passages on the evils of fornication.
And if we wish to destroy a human group, we try to penetrate its boundaries: to defeat an organized crime gang, law enforcement officials try to plant a bug in its headquarters, to place a mole in the gang, or to convince its accountant to reveal its financial secrets.
Things are no different with the nation-state: its survival as a social grouping depends on its ability to regulate what passes through its boundaries.
Of course, anarchists, believing the nation-state to be a malignant social group, will therefore be in favor of "open borders": destroying the ability of the nation-state to regulate what passes through its boundaries will destroy the nation-state. Advocating "open borders," thus, is perfectly sensible from an anarchist perspective.
But for those of us who are, perhaps, not thrilled with the nation-state, but suspect that it is superior to any option presently on the table for politically organizing human societies, there is no reason to support their attempt to destroy it.