This.
https://www.marxists.org/subject/economy/authors/pe/pe-ch40.htm
"Communist society passes through two phases of development: the lower phase known as Socialism, and the higher phase known as Communism." That's from the writings of the Economics Institute of the USSR.
As Brabus said, it's merely a pit stop on the way to a communist society. Even Marx and Engels didn't really distinguish between Socialism and Communism in their writings. To them, it was all part of the same project.
I'm not even convinced anymore that the Left thinks it's a good, workable economic system. I think that's a mask they use to hide envy and a misanthropic desire to destroy things that they can't have or otherwise haven't achieved. It's a hope, desire, and bet on future Schadenfreude.