The free migration of people allows them to quickly become parts of and expand the most productive orders thus vastly increasing production and technological growth. A person in Africa is stuck in an unproductive order which lacks good laws (respect for private property of course), roads, advanced tools/factories, large groups of already-highly-educated and productive minds/order so he produces wealth at a rate of say $5 per day. By just moving to a more advanced/productive country/area like the USA, even without knowing English, he can do many jobs managed/aided by a bilingual English speaker and easily increase his rate of production to say $7/hour or $56/day, an 11-fold increase in his rate of production, benefiting his life tremendously and that of existing American/world producers because they now have additional customers that can actually afford their products instead of poor Africans that can’t. In countries like China, every day thousands of relatively unproductive farmers who are producing wealth at a rate of say $4/day are moving to cities to join/work-with/use factories which might triple their rate of production to say $12/day. Similarly, one of the reasons why the USA has been far more productive than the similarly sized Europe is because people have had the freedom to easily move to the areas/companies that are more productive and there have been no internal tariffs and other impediments to free-trade. When countries allow foreign investment, they make it profitable for wealth to be shipped to poor countries to create the tools/factories/order that can increase the productivity of workers, but it is even better when the workers have the freedom to quickly move to and incorporate themselves with the already existing more productive order.
This is obviously a massive subject with important cultural dimensions… But for now… Bryan Caplan, Professor of Economics at George Mason University is one of the most eloquent educators about the numerous benefits of open borders.