Why would Russian troops in Ukraine fly the flag of the USSR? And why would the Russian MoD advertise this?

From this video which appears genuine:

I'm miffed at why the Russian soldiers are flying the flag of the USSR. And why would the Russian Ministry of Defense advertise this?

A little more search finds additional images, from other sources

Also, commentary like:

A Soviet flag was even spotted on an armoured vehicle in a video from the Russian Ministry of Defence and broadcast on their television channel Zvezda, identified by the red star logo in the top right of the video.