I think that in a utopian world that no borders would be needed. However, we do not live in that world. Take this extreme unrealistic example:

What happens if you live in a country that is among the freest, most democratic and successful in the world except that it has very strict border controls but most of the rest of the world is controlled by Nazi Germany? Then what happens when you remove all those border controls?

Obviously this is a ridiculously exaggerated example but I think that to a lesser degree the point applies. There will always be those in the world that will seek to do others harm. Perhaps because they are racist, perhaps because they seek a theocracy, perhaps because they are seeking their own gain at others expense or for some other reason. As long as people, groups and nations like that exist, borders will be necessary.