Why Does Moscow Have Four Commercial Airports?

With the opening of Zhukovsky Airport on May 30th, 2016, Moscow can officially claim four international commercial airports. Sheremetyevo in Khimki to the northwest, Domodedovo to the southeast, Vnukovo to the southwest, and the latest addition, Zhukovksy, slightly further north and east of Domodedovo. But just how do they differ, and why does the Russian capital have so many?

This is a companion discussion topic for the original entry at https://simpleflying.com/moscow-4-commercial-airports/