Why liberalize immigration?

“Indeed studies suggest that gains to the world economy from removing immigration barriers could well be enormous and greatly exceed the gains from removing trade barriers. For example, Bob Hamilton and John Whalley provide estimates that suggest that the gains from free migration of labor would more than double worldwide real income, indicating that immigration controls “are one of the (and perhaps the) most important policy issues facing the global economy. Even the most conservative estimate suggests that the gains would be a significant fraction (over 13 percent) of the worldwide real income. Furthermore, their analysis indicates that the free migration of labor would also greatly improve the global distribution of income by raising real wages dramatically for the world’s poorest workers.” Chang, 373