It’s because where the money goes is where it circulates, which ultimately affects the GDP.

Every dollar that leaves India, Nigeria or any other developing country is one less dollar that can pay for jobs (plumbers, electricians, doctors etc) in the local market.

South Africa used to have very restrictive currency export controls partly due to apartheid-era sanctions, and one consequence is that it stimulated domestic production and innovation, perhaps far more than any other African country. They’ve now relaxed those controls but it is arguable that they were necessary at that stage of their economic development.

>why would it matter where you're sending a portion of your so-called "wealth" when you're getting something in return?

Perhaps the "wealth" you're sending and what you're getting back are qualitatively different and you have reason to prefer one over the other. Or even more simply, perhaps what you're getting in return isn't worth more than what you're giving. As an aside, in the philosophy of exploitation, it is entirely possible for exploitation to exist even if both the exploiter and the exploited benefit in some way in the transaction.

> considering the horrific state India is in (high corruption, high poverty, low healthcare, no infrastructure, you name it),

You missed jobs in that statement, Iphone factory brings jobs directly and indirectly in that region. Try looking at the big picture.