Borders are analogous to property lines. Both are social constructs which exist only by convention. Both nevertheless exist because Darwin rewards people who practice their use. (And punishes those who do not.)
Property lines exist when individuals claim territory and succeed in defending it.
Borders exist when groups claim territory and succeed in defending it.
If property lines are the boundaries between property holdings, borders can be thought of as the boundaries between property regimes.
In practice, you have the property and property rights that the people around you are willing to concede that you have. One man cannot stand alone against the world.
But a few in confederation can hold the looting hoards at bay indefinitely.
Property and property rights are obtained in exchange. You recognize and uphold mine and I’ll do the same for yours.
But to this basic condition, others can be added to protect the long term viability of the confederation which gives it force.
And first among these must be “don’t let the looters in.”
One condition people may demand for recognizing and upholding your private property is help in upholding shared borders.
You don’t have to assent to that condition, but if not, you might find your position very lonely.