SOYMB was amiss in not mentioning that Sunday was Open Borders Day
The struggle for free migration is an important human rights issue. Your freedom to change your place of residence should not be restricted merely based on the fact that the you were born on the wrong side of a line on the map. Immigration restrictions forbid people to live and work where they wish purely based on arbitrary circumstances of birth for which they are not responsible. They also restrict the liberties of native-born citizens who wish to engage in social engagement with migrants.
Hundreds of millions of people live in countries where their probable fate is a life of poverty and oppression. Many of them could escape that terrible fate if only other countries would allow them to immigrate. Enormous numbers of people currently live in poverty not because they are unable to be productive workers, but merely because they are forcibly prevented from working.
Many potential migrants are also trapped in societies where they are denied basic human rights, such as freedom of speech, religion, and private property. Many of the women among them reside in societies with gender-based oppression and discrimination. For hundreds of millions of people living in undemocratic societies, emigration is their only realistically feasible way to exercise political freedom.
The United States in fact had virtually complete open borders until the enactment of the racist Chinese Exclusion Act of 1882, and open borders for nearly all non-Asian immigrants until the 1920s. In the UK it took until the passing of the 1905 Aliens Act to impose controls and then later immigration controls of the 1960s and 70s to restrict ex-British Empire citizens from settling.