Practically Speaking, Why Would It Make Sense for Already Developed

Practically speaking, why would it make sense for already developed countries to invest in developing countries? Or wouldn't it?

A) Economics-buy low and sell high
B) Helping a developing country is an ethical obligation
C) Helping them develop will slow global population growth for the good of us all
D) It wouldn't. We developed ourselves. They can, too.
E) It wouldn't. It would be better to invest in slowing our own population growth so our country doesn't run out of resources.