Who, particularly? In general, immigrants do. And humanitarian crisis response isn't generally driven by economic bottom lines.
There is no evidence that "diversity" benefits anything and evidence to suggest the exact opposite.
Just insanity. Spice of life. Just make sure they're compatible with secular society.
If you look at per capita crime, non nationals are always over represented. You import other peoples problems. "Africans" are not a homogeneous group. They have the same ethnic, national and religous troubles in Africa that anywhere else does. Why do you think Sweden is having gang warfare on its streets?
A combination of poor management, chauvinism, and unrelenting right-wing slanting.