Why is Samara a “backup capital”?

Strictly speaking, they called it the “spare capital” Kuibyshev — that was the name of Samara from 1935 to 1991. Several months after the war began, on October 15, 1941, the State Defense Committee adopted a decree "On the evacuation of the capital of the USSR, Moscow." It was evacuated to Kuibyshev.
The city was transformed into a rear industrial center: 40 industrial enterprises and military factories from various regions were relocated here, along with the State Academic Bolshoi Opera and Ballet Theater, the Leningrad Academic Drama Theater, and the All-Union Radio Symphony Orchestra. The gas, automobile, ball-bearing, oil refining, aviation, and defense industries continued to develop here. Later, the country's "main voice," the radio announcer, was also transferred here. Yuri Levitan, who read out the Sovinformburo's reports. No one in the country knew what he looked like; his identity was kept strictly secret. Immediately after the decree was passed, representatives of foreign embassies, foreign journalists, and top government officials—Kliment Voroshilov, Mikhail Kalinin, and others—evacuated to Kuibyshev.
That same October, another decree was issued, "top secret." Due to the front line's approach to Moscow, it became necessary to provide shelter for the Supreme Commander-in-Chief. Joseph StalinThe bunker's construction took nine months, with 3000 Moscow metro workers and Donbas miners working around the clock. The shelter extended 37 meters underground (12 stories) and was considered the deepest in the world. It could operate autonomously for five days.
The construction was carried out in secrecy; even residents of nearby houses were unaware of it. With the advent of perestroika became known Details: The bunker is located at 167 Frunze Street, beneath the Kuibyshev Regional Party Committee building. It features comfortable quarters with an elevator, a meeting room, a lounge for the leader, and more. The bunker has now become a museum. Despite the declassification, several secrets remain. For example, how and where were so many tons of earth secretly transported during the mine digging? Another: according to the workers building the bunker, two shafts had already been dug by the time they arrived. This would have taken at least a year. So, did Stalin prepare the bunker even before the war?
Be that as it may, the "spare capital" was able to supply the front with weapons, equipment, and ammunition, with which the country defeated fascism. And then—it won.

