This is the one that makes me wonder for a while.
Big countries' capitals often have two airports - one mostly for domestic flights, one for international flights. Tokyo has HND and NRT, Tehran has IKA and THR, Buenos Aires have EZE and AEP, Brisbane and Sydney have two different terminals from Domestic and International, separated by public transit ride.
Other countries don't have such system. In the USA, ATL or SFO or JFK serve both domestic and international flights and provide connections. In the EU, FCO or CDG do the same. In Moscow, all three main airports serve both international and domestic destinations to similar extent.
From the point of people living in the capital, it can make sense to put domestic airport closer and international airport further away. However, from the point of people living (or travelling to) outside capital, this would become a nightmare, since they have to waste a whole day on airport change, with a costly taxi ride or worrying about a shuttle that may get stuck in traffic. Even if there's a public transit ride between them that's a lot of inconvenience for somebody without local currency, with a lot of bags, etc, etc.
What was the historical motivation for such system? Is it still employed anywhere when building brand new airports?