To progress from late stage capitalism to the promised communist utopia, Marx & Engels envisioned a transitional phase during which many of the political & economic structures of the capitalist system would remain as they were gradually phased out. In this transitional phase, all economic and political power is wielded by the proletariat rather than the bourgeoisie & nobility. Instead of immediately abolishing all institutions and social hierarchies following the revolution, these would instead be controlled by working class revolutionaries organized into councils (called soviets in Russian). The final stage of human social evolution- communism- has no need for government bureaus or militaries and other such institutions of power. During this transitional phase, however, they would continue to exist, albeit under proletariat control. This intermediate phase is referred to by several terms, including the “dictatorship of the proletariat” (a term Marx & Engels adopted from an earlier theorist) and “socialism”.

Semantic note: in classic Marxist ideology, this is what differentiates “socialism” from “communism”. Socialism is the “Diet Communism” phase used to transition society from capitalism to communism.

The dictatorship of the proletariat is what most people envision when they think about communism: Five Year Plans, bureaucracy, large standing armies, et al. The Marxist-Leninist systems of the 20th century effectively got “stuck” at this socialism/dictatorship of the proletariat phase. So, nearly everything that the founders and maintainers of the Soviet system did was to build and perfect a DoP with the long term goal of using these engines of power to effect the transition to communism.