Why does the world have borders?

I think we have borders to keep us safe but borders also separate us and keep others put. I know after the 9/11 attack the borders were strengthened to prevent any other attacks. That's good, because they want to keep us safe right? But then the border keeps people out. What if somebody doesn't have a passport and they want to go to the United States to have a better life?The border also seperates us from other countries. The United States is sometimes considered a little better than most counries. A place you would rather live in. So if the U. S. is so nice then why can't others come in and have a better life? But separation could be good as well because if there weren't any borders there wouldn't be different countries and then there wouldn't be an organized government.